Changing employer

[RDR]

I am working for employer A. My petition with employer A is valid till 2018.

I am planning to join employer B. Employer B is asking me to join as soon as I get receipt from USCIS. If the petition from B gets rejected, can a new petition be submitted by B ? If yes, will the new petition be cap exempt?

If B's petition gets rejected and I go back to my home country, can any other employer file a non-cap petition for me ?

If B's petition gets rejected, can I continue working with A?

If B's petition gets rejected, should I leave US immediately?

Thanks in advance