You can add non-steam games to steam if thats what you mean. Theres a launcher called hydra which does, things. Playnite is a launcher I use that I plug all my other accounts in to and launch all games thru that. There are also ways to unlock dlc and stuff but that will require a little bit of research. Havent sailed the seas for games in a while.

TLDR; Yes you can add otherwise aquired games to steam. For the most part

You mean this?

Control scheme/keybinds, better networking for multiplayer. Its a complete recode full of QoL changes not just a simple remake. I think they also added decent modding support as well given how the RoR2 modding community is.

Because the tether for the quest isn’t a physical one but a digital one.