There's a Github discussion post which is all about that topic. Worth checking it out for advice.
That's interesting! The concept of virtual communities could have some cool uses. I'd love to see that be brought into Lemmy itself so it can be a concept everywhere.
Yes. Just like users, communities are defined by their fully qualified names. So Games@mydomain.com and Games@yourdomain.com are different communities, even if the "Games" part of the community name is the same.
There's no way to like "merge" it though if that's what you mean. So it's possible you will have two "Games" communities you subscribe to for example. However, that's not dissimilar to what happens on reddit already. /r/Games was born because /r/Gaming was going in a direction some people didn't like. And then /r/TrueGaming was born because they didn't like either of those. And you have small subreddits closing to point to larger subreddits, etc. Over time that's a problem that will just naturally sort itself out, though the UI for creating/searching for communities could use improvements to help with that process too.
I think it's almost necessary. I only have to remember one password now. Bitwarden has apps/extensions on basically any device/browser I've used that integrate well with auto-fill. It was weird not being able to "know" my passwords originally, but it's great not ever having to remember which variant of a password I might have used. Plus, you can easily share some accounts with people easily and it's just seamless (a lot of IoT devices only work with a single account for example).