Your "to hit" bonuses are correct, but you may be overestimating Feral Senses
Your "to hit" bonus calculations are completely correct. As you indicated, this tactic will use up six very rare magical items (five +3 arrows and one Oil of Sharpness potion) in order to hit each member of the party once. That seems a rather expensive way to make a first impression, but I'm sure they have their reasons.
That being said, I wanted to warn you that the Ranger's Feral Senses have a couple of caveats that might complicate your strategy. You mentioned that "the players’ invisibility is ignored for purposes of these attacks due to her Feral Senses feature." If you meant that the attacks wouldn't have to be made with disadvantage, you were completely correct. But Feral Senses doesn't let you actually see invisible creatures: it just makes you so capable that you can still accurately attack creatures that are invisible. I mention the distinction because the rules on unseen targets state that:
When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss (PHB, p. 194-195, bold added)
Feral Senses will always remove the disadvantage on an attack roll against an invisible creature, which will help quite a bit. But it only lets you know an invisible creature's location under some specific conditions. According to the text on Feral Senses (PHB, p. 92, bold added)
You are also aware of the location of any invisible creature within 30 feet of you, provided that the creature isn’t hidden from you and you aren’t blinded or deafened.
You indicated that this tactic would be used as the PCs entered " a large hall," and that some of the PCs "are very good at Dex saves." This led me to believe that there are two possible confounds to your strategy.
1.) The Ranger might not know where Hidden (read: stealthy) characters are
If your characters are "very good at Dex saves," I assume that at least some of them may be stealthy. If the entire party is invisible, I assume that at least some of them are trying to remain silent as they move along. And since invisible creatures can always attempt to Hide (PHB, p. 177), if they roll better on their Stealth than your Ranger does on Perception, they will be hidden. And since Feral Senses doesn't let you know the location of hidden creatures, that means your Ranger will be guessing their location, or may simply not know they are in the room. This could lead to your Ranger missing these characters, in spite of his/her incredible bonus to attack.
2.) The Ranger may not know where characters are if they are far away in this "large hall"
It is accepted that unless characters are actively Hiding, it is fairly obvious where they are while they are in combat. However, this principle becomes questionable at longer ranges. If your Ranger is 100+ feet away from the PCs, it's a bit surprising that they can tell where the characters are from sound alone, especially if the characters are attempting to be quiet. Feral Senses explicitly lets the Ranger know the PCs' locations if they are within 30 feet. But at that range, it seems to me like there's a danger that the PCs will chase the Ranger down.
Naturally, as the DM you can declare that the Ranger is aware of the PCs location at any range you want: perhaps the Ranger is wearing a Robe of Eyes (DMG, p. 194), or has preternaturally acute hearing. And as the DM you are completely in charge of when stealth can reasonably be applied, and under what circumstances creatures are capable of hiding. But as you mentioned that you wanted "to do this 'fair and square' with a RAW-built NPC", I thought you should know about some of the RAW limits on the Ranger's Feral Senses.