1) It depends on how you access the lib. If you embed it in your addon (preferred) each addon gets it's own per-target set of callbacks. If you get a direct pointer to the lib via LibStub(...):GetLibrary(...) then the last event hander wins (still per-target, per-event of course).
2) I don't think you should expect any strong guarantees about this. Remember that the messages have to go out to the server and back, so it's very possible that the order could get garbled even if the event sender dispatches them in a consistent order.