No, that's not legal. Trying to back out of this deal because market fluctuations caused the company to be worth less than he wanted would be very, very much against his contract.
He can only back out is Twitter's securities filings are so wrong that profit projections are more than 40% off (MAE standard) or he was unable to secure financing. Spambots being a bit higher than projected is not enough to break the contract, especially given that Musk waived due diligence and that the Twitter filings have always said that the bot number is an estimate. Elon Musk's lawyers are probably more than a bit panicked right now.
Doubt it. They have a client who makes things difficult, but that's nothing compared to the stress of being in a job where a wrong decision costs lives.
I think you underestimate the human ability to feel stress. I recently heard about a video game developer/director having a stroke due to stress from extended crunch. A video game! No lives at stake there. Heck, you should have seen how stressed I was in grad school ... I was nearly suicidal on occasion. But absolutely nothing substantial hinged on the work I was doing.
1. Announce buyout (priced at a premium, of course) is "on hold"
2. Stock drops due to bad news
3. Re-negotiate for lower buyout after manipulating the stock to a lower price