the problem with first come first served is that the awareness of the Summmit and even Maemo is changing every month significantly. I would accept the clear candidates in July or even later as they come but still leave chances for late submissions. Also because lightning sessions are a way to accommodate interesting proposals that by no means fit as 30' sessions anymore in a full schedule.
Also, deadline for submissions on the 3rd of July and then 20 days to start having approved sessions? Isn't this a bit old school? I don't see any problem in approving clear proposals asap as they come, decline the clear weak candidates also asap and have a bunch of proposals in the limbo waiting to see if they end up in the schedule or not, depending on whether stronger proposals arrive later or not.