This sounds like a really fun campaign design! I've run some politics-and-intrigue games (though not in Rolemaster) in which I've needed some similar dynamics, and some of the solutions I found might suit you here as well:
It's easier if the goal is not just about the MacGuffin
Many, perhaps most, TTRPG plots involving MacGuffins have a basic setup of "you have it, but we need it, so we'll be taking it." It's true that there are often other details that make these armed robberies (or burglaries) more palatable for more heroic characters, but it's difficult to prepare a story about getting a crucial MacGuffin without encouraging players to do whatever they can to get it as soon as possible.
But if there are extra, explicit layers which are also important you can do a lot to constrain your players' enthusiasm for simply getting the MacGuffin. Some which I have used include
- The MacGuffin is necessary, but not by itself or not right now. It
may be crucial to possess it before the year is out, but not
necessarily this very day. Or maybe it only matters when combined
with the MacDuffin and MacRuffin, and so getting one of those might
be equally meaningful at this stage of the campaign by denying the sorceress the complete set
- The MacGuffin is important to the things the players are focusing on,
but is tangential to the rest of the game world (early on, at least).
If the sorceress has prepared false evidence on a dead man's
switch that will plunge the continent into war if she dies, your
players maybe still can fight her head on but might prefer not to
- There is something wrong with the MacGuffin. Maybe taking possession
of it also places you under a curse, or something similar, and so
while the players still need to get it they may prefer to plan the
circumstances around obtaining it much more carefully
- The players don't actually care about the MacGuffin, but they operate
in a setting which does. This sounds fairly close to your actual
situation, as the powerful figure who has hired the sorceress to get
it doesn't really care about how it falls into his hands. The players
may have different plans for it, but if the ultimate result is that
the same NPC gets the MacGuffin anyways it's just a question of
whether or not they can deliver it successfully after killing the
sorceress but before being killed by the NPC's minions-- it may not be worth the risk just now
It's easier if the groups are in conflict for reasons other than the MacGuffin
If both groups are looking for the same thing, they'll be following up on a lot of the same leads and going to a lot of the same places. Instead of fighting over the actual MacGuffin itself, maybe they end up in conflict over finding the right guide to take them to the place where the MacGuffin is rumored to be.
Or maybe the sorceress has been planting false rumors about the party accusing them of being violent thieves, and the players not only have to overcome that (due to her head start on them) but they also may not be able to just kill her and take her possessions because after that they'd never be able to shake those rumors. Indeed, killing and robbing the sorceress might essentially make the rumor true!
Perhaps it's not obvious that the sorceress has been hired to find the MacGuffin, and so the party runs into her party (with her in disguise) as a group of humble fellow travelers who happen to be headed in the same direction for a while. There isn't any outright hostility, but tensions rise as it becomes clearer that everyone is after the same goal and they can't all succeed.
The goal of an approach like this is to obscure the fact that the MacGuffin is both the climax of this leg of the adventure and also the immediate source of conflict way in advance of reaching the climax. While the groups may come into conflict before the end of the adventure, those conflicts can be for lower stakes which may not be worth killing (or dying) for.
Pieces of a puzzle
Finding the MacGuffin can require assembling lots of information: old legends, fragments of maps, magical keys scattered about, or similar things. In such a situation, killing the sorceress may be a distraction or even counterproductive. If she's heard one of the old legends from the last living person that knows it, and then killed that person afterwards, killing the sorceress might guarantee that the party cannot succeed (because they will always lack that piece of information). Or maybe it just saves time to let her hunt down different items from the players, as the fact that one group will eventually need all the pieces will force them into conflict at some point as long as either group holds at least one piece.
The NPC and PC parties are similar, and so have to do similar things to find the MacGuffin with similar constraints and obstacles
Fighting with the PC party doesn't put the MacGuffin into the sorceress' hands, but does cost her time and resources. This might give some third adventuring party the chance to find it instead of her or the PCs! Instead, the adventuring parties might be better served by doing all they can to obtain the MacGuffin and only interacting with the other groups indirectly-- laying false trails, installing their own booby traps in dungeons, placing decoy plot objects, or similar things.
The main idea is that direct conflict between groups is inefficient for them, and if they're under time pressure that can be a big deal. If you convey to your players that indirect competition is possible they can still enjoy opposing the sorceress' party without actually getting into a fight.
A thorn in the paw
Finally, a cautionary note: the more you allow your players to interact with the sorceress directly, the more likely it is that they'll kill her. It's extremely tempting to come up with a scenario in which it's a bad idea, or it is extremely difficult, to kill your NPC. In my experience, players love disrupting that setup more than nearly any other activity at the table.
It is extremely difficult to convince players that they can't (in the sense that they really, really shouldn't) kill an antagonist NPC that is evil and directly competing with them for a zero-sum prize which is also the culmination of the adventure. If it's really important that she survive this phase of the adventure, then it's important that the sorceress not be around the PCs any more than is absolutely necessary.