I would probably expand “take in Steve’s stuff he finds acceptable” to include members of Bob’s servers. Local communities are often vocally invested in the policies and standards of their server, and I foresee a lot of friction arising around migrating post archives that were acceptable in one server, into another server where some non-trivial portion of the posts run afoul of local standards.
In fact, I would list the receiving community as a stakeholder distinct from the owner/admin of the receiving server—not least of all because admins and the administered aren’t always in accord on these things.
Along similar lines, I wonder if maybe the departing community might not also have some stake that we’re not fully anticipating here. One question that occurs to me is: How are posts that were initially addressed to only the local community handled? The Hometown and Glitch forks of Mastodon, for example, have options to restrict federation on posts so that they only appear locally. Local-only posts are sometimes used to discuss internal community business. What happens when Steve tries to migrate those posts to Bob’s server? Technically, they will become local to the receiving server. Does that make internal communication from Alice’s server visible to members of Bob’s server?
Along similar lines, what about DMs. It’s fairly well known that DMs on most AP implementations are insecure, and that a sufficiently motivated server admin could read those ostensibly private messages. Third-party trust is an inalienable factor in those communications. So maybe Ingrid trusted Alice when she DM’d Steve, but she definitely does not trust Bob, who now stands to gain access to those DMs.