Powerful NPCs deal with Tier 4 Threats
As a parallel answer to @ShadowRanger's excellent treatment of where the NPC's attentions are directed, this answer is broken down into two parts:
Meta and world building concerns, and
How to weave this into your commendable efforts at coherent world building so that your plausible denials - "No, Party of Adventurers, I cannot spare any more resources for you" - resonate somewhat better with your players.
In your players' defense, it never hurts to ask - but they are the problem solvers. The DM / world can tell them "No!" with good in-world reasons.
The conversation can go something like this (thanks, @RBarryYoung):
PCs: "Can you help with this problem?"
Powerful NPCs: "We are helping; we are sending you to deal with it."
PCs: "Can you provide any {additional} resources?" {Beyond the potions or item already provided)
NPCs: "You are the resources we are providing."
I'll offer special treatment of Artus Cimber near the end; he is a walking anachronism. Cimber and his best friend are AD&D 1e characters (from a Forgotten Realms novel) who are alive and well in D&D 5th edition thanks to a magical device1.
The World Building Piece and Sequencing
For what it's worth, Tomb of Annihilation was published before Waterdeep: Dragon Heist; they were both published as stand-alone adventures. Running them out of order is fine, as a DM, if you take the adventures through level 5 in Waterdeep and send them off to Chult. But the two of them were not put together as a coherent 'world building' exercise by the publisher. They rely on the "we leave it to the DM to fold this all into their instance of the Forgotten Realms" approach.
Rule 0 of FR DM'ing: there is no continuity without holes large enough that you could sail a galley through them unless you, the DM, patch them up.2
Your ToA PCs deal with Tier 2 and Tier 3 threats
Your party has already handled a Tier 1 adventure.
- The high level NPCs deal with global threats to their own domains, their own factions, and their own interests as well as with higher order (Tier 4 and beyond) threats to both the world and the multiverse. The ToA adventure winds up as a late-Tier-2-to-early-Tier-3 adventure by the time it is over (per the guidance published in the book). Let's look at adventure tiers to see what's going on here.
In the second tier (levels 5–10)... characters have become important, facing dangers that threaten cities and kingdoms.
In the third tier (levels 11–16)... mighty adventurers often confront threats to whole regions and continents.
At the fourth tier (levels 17–20)... the fate of the world or even the fundamental order of the multiverse might hang in the balance during their adventures. {Basic Rules, p. 12}
As you have correctly diagnosed, the adventure tiers are muddled somewhat by Acererak's aims. The potential negative effect (beyond Toril) of him achieving his aims possibly hit Tier 4 (fate of the world or multiverse) in their implications. The PCs can stop him within the lower Tier and level constraints of the published adventure, however. His schemes become more important
if you use the ticking clock for a thing to happen before they confront him in the ultimate battle and
in the near term for just those NPCs who have had the benefit of 'raise dead' or other resurrection magic, which is expensive and well beyond the reach of most of Faerun's population.
This means that there is a small population who care but who may or may not have a clear idea on just how big this problem is. To complicate matters for these high-power NPCs, most of them have lots of other problems to solve - and then this threat gets added to the mix. That's why they delegate this task to adventurers - the high level NPCs are out of time and resources.
- If the PCs can handle this late-Tier-2-to-early-Tier-3 challenge
(levels 10-12 as suggested by the adventure) they have made powerful
allies for future adventures. If they fail, and die - well, it's no
longer their concern (in-world).
The problem in Chult is one problem out of dozens
You can posit (in your own mind, and by spreading rumors of big happenings in distant parts of the Realms) that, beyond the urgency that the local leadership in Chult feels,
- The Storm King's Thunder adventure is also ongoing at roughly this time (there's a related hook involving some Giants in the ToA adventure) - this itself is a non-trivial, world-sized threat that the same NPCs likely will be aware of. Drop hints and rumors about this threat to the Sword Coast.
- There are Kingdoms all over Faerun that are not on any map published to date in D&D 5e (Thay, Kara Tur, Cormyr, other continents, Maztica, et cetera) each of which can pose threats to the slice of FR that your PCs are familiar with: The Sword Coast. The Sword Coast's problems, versus those of Toril, are in proportion to the United States East Coast, and its problems, as compared to the real world and its global problems. The High Level NPCs have to address world-level problems as well as local problems. The PCs are handling a local problem (as seen from the capital city of Chult, or from Waterdeep, anyway).
- Extraplanar Problems Abound - this ToA thing is just one of them. If you also position the Avernus Adventure ongoing in parallel with this one, and consider "continuity wise" the mind flayer incursion in the related product (Baldur's Gate 3, CRPG) and any high level aspects of the Elemental Evil, Out of the Abyss, and Rise of Tiamat adventures, these also pose some serious threats for the Sword Coast and regions beyond. Rumors of these calamities need to crop up no matter where your players are when they hit a tavern, city, public square, etc. Every other part of Faerun on all of its other continents has something similar going on, or does potentially.
Resource Management: PC parties are a great value to a leader
Most of the NPCs you mention were once adventurers who know the high 'bang for the buck' value that adventurers offer. The NPC's have world-encompassing, and extraplanar, problems to cope with - of which the ToA is just a piece - and they have finite resources. An adventuring party represents a high value proposition to these NPC's, who know from personal experience that they can apply their other resources to a myriad of problems before they are tapped out.
Why?
For a modest cost (the items and gold you mention) they get huge benefits from the PC party while they handle their already sizeable load of problems.
Yes, these high level NPCs are using the PCs! That's the harsh reality of the adventuring life. Someone always wants to get as much done as they can, as cheaply as possible! When you treat the high level NPCs as politicians first, not as adventurers, it becomes clearer. Beyond that, these high-powered NPCs may underestimate how dire the threat is. They don't know what the DM knows, or at least they don't know all of what the DM knows.
NPCs are not omniscient!
Being politicians, they may also have perception and priority problems vis a vis "what's the biggest problem I need to solve right now?" 3
A modest example at Mid Tier 2 (level 7-8 at present)
My current Saltmarsh party (that I DM for) keeps running into the regional political problems - they more or less trip over them as they undertake various adventures, and they are beginning to get caught up in the inter-faction conflicts as they conclude the Final Enemy arc. (One player kvetched about getting tied up in politics, and my response was "That's a part of this campaign, and it's not going away!" He has adapted pretty well.) These regional problems have been alluded to since the first session (level 1) when the Sorcerer was kicked out of Monmurg (Sea Princes area) as part of why he ended up meeting up with the other adventurers. The druid PC (at level 4) heard rumors of a war (Lizardfolk were arming) and he came west to try and prevent one. The cleric was assassinated (at level 4; the player had to drop out in RL; we worked out that decision together).
And so on.
The party has discovered that local political tension with Seaton informs the constraints on local NPCs, as does the limited budget from the King of Keoland for any risky operations, and they have also learned that the Night Hag (Granny Nightshade) threat from the Dreadwood limits the assistance of the powerful druid near Burle.
They also tripped over a regional / world-level threat (though I am not sure if they appreciate that yet) as they concluded the Isle of the Abbey adventure.
The bottom line is that the PCs are asked to solve the problems that are roughly within their scope.
NPC's Line: "Solve this problem, I've got bigger ones"
That's your theme. Repeat it ad nauseum if need be to get the point across. The PC adventure party can't handle the whole world's problems, plural; that's what the high level NPCs try to deal with - with some successes and some failures. I have wargamed, on the side, a few related 'what if' scenes as a result of player success in Salt Marsh and have kept track of the results. That also has informed play. (@Shalvenay helped me with one of those scenarios.) The PCs handle one (significant) problem so that the NPCs can handle (or try to) the rest with their limited resources.
I can't stress this enough: the high level NPCs have limited resources compared to their responsibilities and challenges. They may appear to PCs to have infinite resources, but they do not. PCs are a cost effective way of spreading the NPCs resources out to best effect in order to solve multiple problems at once.
And for that matter, returning to FR, the Zhentarim are always causing problems for everyone on the Sword Coast, right? They are a never-ending resource drain for most of the high level NPCs along the Sword Coast.
Artus Cimber: the special case
Artus is very much a fish out of water, as are those hunting him to a certain extent. My suggestion to you is to play him as being obsessed with one goal to the exclusion of all else:
Find Mezro and find his lost love.
Play Cimber as being 'stretched' due to his long association with that artifact: from a narrative sense, this is sort of like Bilbo telling Frodo how he didn't feel old, but stretched, by his long association with the One Ring. He lived well beyond his expected life span, as has Artus. The longer Artus lives, the more obsessed he gets with recapturing that one, overriding goal (in the spoiler box up there). Call it tunnel vision, call it the madness that love brings to us, call it what you like. He's become eccentric enough to become a liability to himself and anyone around him until he finds what he's looking for.
Anything the PCs ask of him is barely going to resonate. Before the story is over, he may just wander out of Chult if he gets a lead that takes him where he wants to go, or, as @nitsua60 (from RPGSE) had happen in our ToA campaign (I was a player) he gets captured by some giants and exits stage left in their custody.
Both of those options are plausible narrative ways for Artus not to become a DMPC who overpowers the party. The "Bilbo gambit" is what a friend of mine used for Artus when he ran this last year; he came to me for advice and this is what we worked out. His comment was, after the fact, "it worked but it still felt clunky - Artus being in that adventure was not necessary".
One last example from an old campaign.
Our party made the choice to try and crack down on the Slavers of the Under City. (AD&D modules A1 - 4). Our sponsor had alluded to other problems with the Temple of Elemental Evil, and a couple of related (homemade) bad things happening up north. We got back from taking the slavers down to discover that Verbobonc had been flattened by a hill giant raid, and our sponsor was taken captive. He was in no position to provide us with our reward for shutting down the slave ring. Freeing him was the hook for us to go after the Giants (G1, G2, and G3 modules) - in that case, his larger problems got the better of him. You can apply this to your ToA campaign similarly, given the wasting-away nature of Acererak's overarching plot. The NPCs can die, or fail.
NPCs are not guaranteed plot armor unless you, the DM, give it to them. 😊
1 It's a little like The One Ring from LoTR. That item Artus has gets its own sub arc in the published adventure that you can use to remove Artus from play, or, to have him lead the PCs on a wild goose chase due to being pursued while he tries to achieve the one goal about the one thing he cares about.
2 This answer does not address Adventurers League attempts at continuity, but covers your problem with your world building and plausible motivations for NPCs to say "No!".
3 No digression will be undertaken to RL parallels vis a vis politicians.
