Hi! Answering the questions:
1) Yes, you can use my assets in your game that you sell on Steam (even the free one)
2) There will be no problem, but if you use free assets, it would be nice to provide a link (or credits) to the assets :) (you don't have to)
3) No, credits are not required
You can change asset pack as long as you don't sell it.
I hope I helped Thanks!