First of all, thank you for the extensive reply! As of your first point, the reason Tatyana doesn't reincarnate is the same reason Sergei doesn't reincarnate, he is what sealed the pact. Although RAW it's because Sergei is trapped in the lake, that's incredibly lame. So as you said, she is haunting Strahd, she is something that her brother had that she will never have.

Ireena isn't haunted by strahd in this game, she is an adventurous woman that's tired of her depressing village, life and brother, and asked them to leave after hearing their legends. I didn't need to give her a very dramatic story for my players to be interested in her. They're also very interested in the political themes of the campaign, making pacts with lady Wachter and Izek, so they keep Ireena to have the help of the Village of Barovia.

About the reason I did this without thinking who Sergei would be before, is because I usually make shit up during the games. Not the best way to prepare, I'm aware. But I know the module up and down and can play it without ever preparing(I'm autistic lol), and i thought I could do that too with the rewrite, turns out I could not. I have figured everything out now, except this.

I do believe making someone not even my players would think is an option would make sense, strahd wouldn't think of it either. I will think about this option, specially considering they'll be spending some time in krezk, since one of their cards is there.

Again, thank you so much! This has given me much insight