Expecting every different platform to all move in lockstep is not feasible so a situation where some support downvotes but some do not is going to have to be part of the process. Even if it was just kbin to kbin initially, that'd still be an improvement.
Yea, that is what i was thinking. I myself am working on some ActivityPub stuff and while compat is important to me, i also plan on potentially exposing new features in this manner. If other software wants to federate with it, cool, if not, whatevs. It's data propagation, as with everything in ActivityPub it's optional that consumers use it as desired.