@silverpill sorry to butt in, but all of this would be solved by respecting what the canonical representation of a reply thread is: the replies collection of the top post[1]. I think it makes much more sense to built that correctly and rely on it instead of trying to keep all the nodes of the network in sync with countless activities.
[1] Caveat, this requires that all replies to a post, include in their inReplyTo property at least their parent and the top post - ideally they should include the whole chain of parents to the top post.