Still thinking about running this because I like the core idea so much.
I think Kas and Vecna being jilted lovers makes their story a lot better, and adds a lot to it. I think this human weakness is what Lolth is using to manipulate Vecna, who doesn't want his ritual to complete but does want it to unmake Correllon as revenge for what they've done to her.
The Crown of Lies I almost like; I'll nerf it to be "no one questions who you are" but it doesn't warp every lie. LIkewise, the stress of the sabotaged Wish will cripple Silverhand but not Tasha, who will be an aspect of the Fey Queen Tasha, not Past Tasha.
I love how the Dark Powers are described in this adventure. I think Lolth would be aware, by now at least, of the Dark Powers existing. To that end, knowing they want to claim Kas and Vecna, I think the climax is Vecna being sucked into a Domain of Dread with Kas, leaving Lolth at in the Cave of Shattered Reflections.
I've always loved the end to Watchmen, both movie and comic, when Ozymandias reveals he's already hit the nuclear button. So I would probably have Lolth have already killed Correllon, and is now using the Cave to spread her web throughout all spacetime to become the next Overgod.
I'm 100% using Tiamat as Vegeta. I'd give the players a backstory of having defeated an avatar of Tiamat before, and now they have to ally with Tiamat to beat Vecna (in reality, Lolth). I'd make the Avernus campaign canon as well, so Tiamat has helped adventurers before. I think Tiamat as an anti-hero is way cooler than TIamat as an evil god villain.
The Raven Queen is another figure I'd involve. She, in my universes, controls the Dark Powers; she is their personal goddess, and she wants those secrets to find out who she truly is. Raven Queen in 5E is also an elf psuedo-goddess, so she has stakes in this adventure as well. But the Cave of Shattered Reflection is a great place to have the Raven Queen figure out the truth of her history, progressing that character tremendously.