There's already a class action suit underway regarding Tesla's mass constructive dismissal, so it'll be really interesting to see all this go down. In the US, there are two things to consider: legality, and lawsuits. Regardless of legality, you can still drown in lawsuits.
Yes. The set of conditions that make it illegal to fire somebody in the US are very very small. "I need everybody to crunch 80 hour weeks for months and months" is a thing employers can do for most forms of white collar knowledge work.
The department of labor does. Even here in Idaho it's amazing how excited the legal team gets when sent a letter of inquiry from the DoL. I imagine they have more teeth in CA.