But it's not automatically. I think the developer has to enable that. Probably because the developer is still intended (or wants) to verify that it really works properly in Proton.
Or not sure. But I have not found a way to run arbitrary Windows Steam games on Linux with Proton. Not sure how I would do that.
Yes, you can force it on all games if you want. I'm currently playing many games on the latest version of Proton.
It might be named a bit confusingly though, I'm at work and don't remember exactly but it might be worded something like "Override proton version for all games" which also enables it for games not explicitly marked as compatible.
Or not sure. But I have not found a way to run arbitrary Windows Steam games on Linux with Proton. Not sure how I would do that.