I'd expect the reservation process to run on holiday periods only at first, but since it's Yosemite, you never know.
I doubt that. There would only be two holiday periods left this summer (July 4th and Labor Day).
They're likely to run it all summer and into fall, otherwise people will flood in (remember what happened when they made it free in March before closing?).
Plus Yosemite is busy enough during summer, after being closed for so long there will be a lot of pent up demand for it.