I see his chap 1 bonus scene in the list but don't know how to unlock it...

Btw will the game coming to Steam after fully released? I was hoping it will...

Kav's bonus scene unlocks at the same time as Noel's scene does and will be available once Noel's is!

WSC can be wishlisted on Steam here. I'm not sure when it will go live there - some time after Chapter 10 goes live.
https://store.steampowered.com/app/3946180/When_Stars_Collide/

~Esh