Also: does that mean you’re only ever allowed an account on one instance? If you have accounts on multiple instances (one per instance) and get banned on one, you’re evading a ban by having the others?

I wouldn’t call it ban evasion unless you were specifically using your other account in that way, rather I think most instances will ban all of a person’s known accounts if they’re banning them, but that’s definitely not a hard and fast rule. Pretty sure there are some instances that don’t even prohibit ban evasion.