Tuthrick: It's hard to place Blood Omen 2 in the timeline, but from what I understood the events of BO2 happened, because Kain saved Raziel at the end of Soul Reaver 2. He even says something along "The Hylden, we walked straight into their trap." after he gains new memories. It was possible to change things, because of the paradox (Raziel and his spirit attached to his arm being present).

I think that Kain tried to prevent the events of BO2, especially his initial defeat, but not only that in my opinion. It's not explained and he's quite cryptic about it. Maybe because he tried to change things again looking for the correct outcome, where he and Raziel could live and fullfill thier roles instead, but it seems that he was unaware of what was Raziel's actual purpose. Kain was hoping for the edge of the coin, but didn't know what would happen exactly when it would occurr. That's how I see it.

Maybe the Hylden would use someone else if Janos remained dead, but then maybe this would cause the story of BO2 to be a lot different, especially if they would choose "the walking paradox" Raziel. However, we know that Janos was thier target and that's why they somehow influenced Raziel when he met with Kain in the Avernus Cathedral.