The reward-loop point is the one I'd put first in the proposal itself. A short repeatable format only helps returning players if the first step is obvious the moment they log in, so spelling out the loop in one sentence in the event announcement is worth more than fine-tuning the date. On the metrics question, separating new registrations from returning logins also needs a baseline — compare each event against ordinary weeks, not just against other events, or a busy holiday period will look like success on its own.