When will movie theaters officially reopen in the USA? ?

It would probably be awhile yet.

Probably not until after there's a vaccine.  So November at the EARLIEST.  It would be the height of irresponsibility to go to a theater before then.  I'm definitely not in the 'corona panic' group, but I wouldn't go to the theater until a vaccine is in my veins.

I just read that the next Star Wars movie is being put back a year, and Mulan is on hold indefinitely. 
Clearly, no  big  cash cow,  money blockbuster, is going to be released,   while theaters have restrictions . 

They are not gonna waste releasing  a billion dollar film on half empty houses. 

They are open in some plaqces and not in others. It's up to each state and no one knows when it will happen. It's unlikely to be soon with all the hotspots in the US.

Plan on watching them at home when they are released to streaming markets.

We havent even reached the peak of this crisis yet, never mind got it back under control enough to consider gping back to normal. My guess, the cinema industry is going to crash out completely by the end of the year, to be replaced by ppv streaming service releases, and cinema houses will limp along by showing cult classics, movie marathons and other niche movies to small hpuses of large, luxury seating and dining

As soon as COVID19 is under control.