I was fortunate enough to attend the 2/24 MK event. Had a girls' trip scheduled several months before the Membership Extra events were announced. I was the only DVC member in the group and my 4 friends & I were staying off site in a Marriott timeshare.
Those "qualified" Members with onsite reservations (think it was only a DVC reservation) were able to make a reservation about one week prior to other "qualified" Members. I made my reservation for 5 (could have made it for 6 if needed) on the day it opened for the non-DVC reservation group. Reservations remained open for quite a while after that - at least several days and maybe even a few weeks before selling out.
I honestly do not think anyone who attempts to make a reservation for the EPCOT events will have a problem if they do it relatively close to the day the reservation-windows open. EPCOT can handle a lot of people.
The MK event was a total blast! My friends are still talking about it. The firework show was the best I've ever seen and that includes the special Christmas & Halloween ones I've seen. IMO, Disney hit a bases-loaded home run with the MK event and I can't wait to attend the one at EPCOT. Really lucked out when they decided to schedule an EPCOT Membership Extra event during our normal time to visit.