If an instance you had an account on went dark, you would lose access to that account, and the ability to view your comment history in isolation. As far as I can tell, the comments you made in communities would exist as long as the instance the community is on exists. Same with your username - there isn't a mechanism for making them globally unique beyond appending the instance domain name (again, just like e-mail).
Beehaw is kind of a special case right now because they have chosen to cut themselves off from a number of other Lemmy instances including lemmy.world. Normally you wouldn't need multiple accounts - I read and post on multiple instances from just my lemmy.world account. You can see remote instance communities under "Communities" / "All". If one you are interested in isn't there, you can search for !community@instance, and wait a bit. That starts the sync of the community to your instance, and you should be able to access it a few minutes later.