On point 4, is the general flow that an "update" would read the current latest, check whatever conditions, then rather than updating, insert a new record (all in a transaction), or is your append only sot more an ordered "log" of update requests with the result of any conditional operations requiring a replay (from at least a check point). Or something else entirely.

I'm generally an immutable first, type developer but ive not had to do much schema design for a while and looking back to previous attempts we never hit scale to stress test my approaches.