Unidirectional federation sounds like a nightmare on the UX side, so if that's the case I imagine it'd be Threads to Mastodon, enabling you to follow (but not interact) with Threads users.
Define swap. I'm hearing instances which federate with FB will be defederated as well. If instances need to be federated to "swap", it likely won't be easy.
Our splinter network will exist but be a non-factor to those who want "reach".
What do you mean by this? If you're a Mastodon user, you'd just see Threads users on your feed, and be able to reply/boost/etc their posts normally.