the concern that @andrestaltz brings up:

Quick question: would it be easy to fetch 'about' messages (which are typically early in the feed)?

Is really the most important thing if we are proposing partial replication - the context and meaning of later messages is some times related to earlier messages (for example, whether or not you followed or blocked someone, or about messages...) - so if you are replicating only the latest messages... how do you know you havn't missed something, and what sort of impact does that have on the application? what kind of applications works with this, or is broken by this?

regards out-of-order messages: ooo messages are securely verified to be the messages your friend mentioned. Your friend posts a message referring to a message by someone you don't replicate. So you pull that one message in so that you can understand the context of your friend's message, it's the signature on your friends log that secures it.

If you have two out-of-order messages by the same author, o1 and o2, you can't prove that o2 really signs o1 - but what is that even useful for? you only know or care that these messages exist because of your friend, so I suggest that we've already captured the important thing.