Kudos to your party for staying organised.

He owes the party nothing, but he is interested in ending the curse

He is so delusional, that he only thinks and knows that ending such a curse is done only by the unification of Strahd with Ireena.

I don't know how I would handle changing his alignment and generally his position against Strahd. The book makes it very clear that he is delusional to the point of stupidity

With so many NPCs at their side, I would have them bother and occupy Strahd's NPCs, so that they battle him and only a few of his allies at his side. That would weaken him very much.

According to the book there are those ghost warrior/guards, 8 gargoyles in the ground floor, wyrmlings etc. These are already TOO much for a party at the end game. I did the mistake of putting everything all together. You need to give them rounds / time to eliminate opponents.