Hey there, I know there are a few recent threads on the JR Railpass, but wanted to get some definitive answers.
Our plan: a 2-week Green Car railpass, for use around July 28 to August 11th.
I've done this before, but at quieter times of the year so never had any issue validating the pass at the airport, usually no big lines to wait for booking seats.
But my last time using this was in 2018. Seems like there are some changes, and I might be able to make some online reservations? A few questions:

1) Which website should I buy my Railpass from? Does that matter in terms of being able to do online reservations? I think I've read that certain points of sale are better than others. TBH I don't really understand this (new) feature for railpasses, I last had one in 2018 and everything was done in person.
2) Obon week (one of the busiest in Japan) is just at the end of our trip. We are also hoping to visit the Aomori Nebuta Matsuri, a hugely popular event, and take the train there. Can we make online bookings for seats for these tickets if seats are available? Any other advice on this front of using the Railpass in busy season?
3) Is it still true that all Nozomi and Mizuho are not allowed, so just the "regular" trains?
4) If Green Car is full, can I still get on a train in economy class?
Thanks!