the number should be configurable

I hesitate to create yet another configuration variable. I'm inclined to use the same one as for chat messages, at least for starters.

notifications shouldn't replicate

Notifications from the new watcher mechanism do not replicate. The only duplication you'll see now is when you get a reply to one of your own nodes (old mechanism) and you also have a watch which triggers (new mechanism). Unfortunately, these two mechanisms are unrelated and in separate parts of the code, so de-duplicating here will be ... non-trivial.

I'd prioritize per notified node:

I like this idea.

I have the impression you want to create a fourth category for multiple triggers

Not at all.