I can’t answer the first two, but for the third question:

Yes, there’s a Proton default. If you go into a game’s settings page, you can choose to either leave it at the default or override it.

Proton is in active development, so it changes over time. That means the default setting might be good, but not optimal.

My main gaming computer runs Linux, so I’ve dealt with this. It’s not something I usually touch. I think the only time I’ve had issues is when I’m running a non-Steam game through Steam.