Same here. Newer versions of Easy Anti-Cheat work fine, but pretty much anything else breaks. Rising Storm 2: Vietnam is an example of a game that uses EAC, but with a version too old to work with Linux

Yes, I’ve run several games native. ProtonDB will indicate if it runs natively (though some people will report using proton on natively supported games out of habit)

EDIT: some games are supported natively, but should use proton for mods. For example, Mount and Blade Warband runs just fine without proton, but if using mods it should be run with proton. This will also be indicated on ProtonDB in my experience