@reiver@reiver ⊼ (Charles)
From personal experience, at the very least anything based on @fedifyFedify: ActivityPub server framework can represent multiple keys for an actor.
FEP-521a has a list of implementations: https://codeberg.org/fediverse/fep/src/branch/main/fep/521a/fep-521a.md#implementations
On changing keys, I used to think this was impossible, but then I saw Claire mention that Mastodon will simply accept a changed key as long as the valid updated actor can be fetched from its canonical URI. So I guess that might work straightforwardly?