Not lost, but inactive / isolated. As I understand it, when a user on insurance A subscribes to a community, votes, or comments on a community on instance B, that content is copied to insurance A and the two instances will sync their changes together. If instance B shuts down or the two instances defederate, then the content on instance A stays intact, but it no longer syncs with the source of truth.
Yes, but they would be separate accounts (comments, subscribed y communities, messages, etc). I have an account on lemm.ee and lemmy.world which I actively use. It can get a bit annoying making sure that I stay subscribed to the same communities on both, but it’s also nice to get different feeds.
Another option would be to stand up your own closed instance, so your account is the only one. That way storage and bandwidth should be minimal enough that you can host at home and also have full control over settings on your instance.