Answer 1. Under the immigration law, there is no risk. You have a choice to work with the new employer and also continue working with the old employer.
Just because a second H-1B is approved, it does not mean your old H-1B is revoked, canceled or overruled. They both continue to be in force, but you have to choose one or the other.
Answer 2. Technically, you have violated your status. You should go get your H-1B visa stamping and come back. That is all you have to do.
As for the time limit you are already working without status. Your L-1 is over when you get a change of status. So the earlier you resign the better you are.