The May 1st date is not even close to official. It is being used as a place holder date for many websites... now they seem to be copying each other. I wouldn't count on that date in any way.
While no one knows, it is likely that once it is open, or a date has been officially set to start accepting FP+, all dates in the next 60 days will become available at the same time. For some that means they will only get 1 day to try to grab a fastpass, for others it will mean close to their normal window... and of course re-arranging existing fastpasses to make it fit. It will most likely be 1 night of chaos, but it will effect a relativly small number of people, and in reality there isn't much else they can do without it becoming even more complicated. I would keep a close watch on these forums, because its also unlikely that Disney would announce such things in a public way that you would be able to easily find.
While its true that few, if any(?!?) attractions have opened in the FP+ era, there have been several restaurants, and this is how they handle ADRs. I can't imagine it would be any different.