Don't know criminal law too well, but I'd think that there's probably a principle that you can't consent to head trauma or a similar level of injury. Whether it is a good case to take to trial is another matter, because I'd think there's a decent chance for jury nullification or a hung jury.
I'm not aware of any such principle. Consent to a fistfight is a full defense even from felony battery charges as long as the person doesn't die. And there's a weird wrinkle here, in that California's sexual battery statute doesn't cover the head-punch as it doesn't concern an intimate part of her body. (That said, there's at least one other allegation of hers that certainly does.)
This probably doesn't end well for Bauer. With so many guys before him who got suspended due to MLB's policy, it will be a very narrow tightrope to walk unless there is substantial evidence to prove his innocence.
His agent (who is a lawyer at a well-respected and fairly sizeable law firm) claims he has strong evidence that proves consent. Whether that's true or not is a matter to be seen. Bauer has also claimed that they agreed on a "safe word" and that she never used it; she claims that's because she couldn't speak when she wanted to. This is pretty ugly either way.
The Athletic story describes an ugly, ugly attack on the woman. Court hearing on July 23. Suspend him until then.
Maybe, but it's indisputable (and indeed she doesn't dispute) that after one sexual encounter that was far outside the bounds of what most vanilla people would consider acceptable, she consented to a second encounter. That's where this apparently happened. And yes, what is alleged is very ugly indeed.