> How far back should you be able to amend history? What if a malicious client adds messages to a conversation that happened in the past? Imagine for example I'm at work and notice a critical mistake that I missed, and so I retroactively add messages to the old conversation to make it look like I'm not liable, should that be permitted by the protocol?

I believe that's impossible? At least if you design it correctly.

For ordering/interleaving purposes, what matters isn't the time you claim to send the message, it's the time the message is received by the server. If you want, you can display the claimed send timestamp beside the message (and prominently highlight it if it is e.g. out of order, or with a long delay, etc.), but that is irrelevant to the ordering.

The point here is that there should be a single consistent order on the server, and that's what all clients ought be displaying. Any messages not yet acknowledged by the server should be displayed differently so that users are aware they haven't been seen yet, and any messages that arrive before those are sent would obviously get inserted above those.