What happens with updates for purchased apps after the associated Google account is removed?
All apps you've bought via your Google account are bound to that. Not only they cannot be updated without your account then (which is where they're bound to), but some of them will even stop working if you remove the account (those which use Google License Check to verify a valid license whenever you start them).

And no, there's no way of transferring ownership from one account to another. Though it would be technically possible, Google is not offering that.