Sorry for the delay. I just ask that if it is bad news that you not shoot the messenger. Shooting the messenger is either leaving a negative rating (usually reserved for rude or unprofessional experts) or not leaving a positive rating at all.
1) Had they not revoked their petition, you would be able to go back to A without any issues, but since they did, I would not go back to A until the transfer with B is approved. If B is not approved, then you are in a difficult situation. You can try to use the new regulations that allow for a 60 day grace period to get back into status, or you could leave the U.S. and apply for a new H-1B visa through A's petition if it is approved.
2) As I said, I think it is very risky to leave B to go to A when B's petition is up in the air and so will A's petition be as well.
3) The risk of denial of A's petition would be hire.
4) You really should stick with B until approval then if you want to go back to A, you can.
Every time you make a change to your status, you give USCIS another chance to find something wrong and deny. The less changes you make, the better it is.
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