I was playing pretty fast and loose with the story. If you hadn't gotten the mindworm in your head you would have found another way to survive the fight with Gareth. Any dark magics employed by the tsochar would likely have been epic or homebrewed spells stolen from or donated by random evil mages. This is just the direction I was planning on taking the game based on where we'd been thus far and where I thought I could make the story though- I guarantee you what would have actually played out would probably at least have been a little different. No story survives long after coming into contact with a group of PCs. :P

Tahlion and Kinslayer I hadn't made concrete plans for, they remained fairly minor characters and would probably have been used as powerups somewhere along the way. Assuming they survived the final battle, Kinslayer would likely hang out with Gaudemus for a while before taking off for parts unknown, and Tahlion might have stuck around with you guys a bit or maybe taken a convenient interplanar portal to see how celestia's changed since she was there last.

Nightfang might not have survived to the end of the campaign, I had vague plans for him to be possessed by a tsochar to slow you guys down/piss you off. Given the direction the game was going towards the end there, we can probably assume he would have either survived or been resurrected after that, though.

Vera's actions at the end of the game would have depended on Dae's, though it is entirely probably she dragged him off to some quiet room and did naughty things to him while the victory celebration was going on.