CapnZapp
Legend
First off, you do whatever you want in your campaign. This is a positive threadAnd I believe a climax like that should happen.

That said, let me try championing BryceLynch's point one more time, okay?
If you shove Ilvara in their faces even if the PCs have done everything in their power to avoid meeting her, you are setting a zero value on those actions. This isn't just about an adventure being a railroad. Even in a railroady adventure, the players will appreciate it if you the DM listen to their actions and adjust accordingly.
If you notice they have relaxed a bit about the Drow threat, sure. If they stop worrying about Ilvara catching up, yeah. If they focus on other important stuff, say Demon Lords, go ahead.
But the point isn't "don't ever run that final confrontation".
It's us saying "if the players obsess about not being caught up to, then let them have this small victory".
You could always use another encounter of comparable challenge if you want a climactic "exiting the maw of darkness" battle. The players doesn't have to know there is no way to avoid such a battle. However, if Mistress Ilvara shows up despite their best efforts, they will know all their plans and ideas were for naught.
In the spirit of being constructive, let me make a final suggestion:
You could even have a Drow battle exactly like in the book - simply say Ilvara has used her connections to persuade fellow priestesses to set up ambushes at likely exit points. This way, you can reuse the exact same encounter, only by changing the name of the Priestess of Lolth, you're acknowledging all the hard work the PCs went through to throw her off their track, and you will be giving them the satisfaction of denying her her final moment!
And that's what all this is about: acknowledging the actions of the player's characters!
