H4 to H1 with multiple employers


My wife had applied for H1B Petition in April 2013, got it approved in Aug 2013, appeared for H1 Visa in Dec 2013 and got 221g. Since the 221g was taking too much time to be resolved, I decided to get her to US on H4 visa (I am on H1B currently). The H4 visa got approved in Mar 2014, and she came to US in Apr(1st week)-2014(entering US for the first time). 


After staying in US for some time she got bored and decided to pursue job opportunities, and now she has job offers from two companies, both ready to apply Premium H1 with Change Of Status from H4 to H1 (The previous petition has gone back to USCIS for review and possible revocation, in Mar 2014). 


Since she has been on H4 so far, she doesn't have SSN yet. 


My questions are : 

1a) Assuming both the petitions are filed simultaneously, what are the chances that COS will be approved on both. 

1b) And if the above happens, which company is she supposed to join ? My guess is the last action rule will apply, and she will have to join the company whose H1 with COS was approved later. Please confirm.

1c)However, say company A's H1 is approved 10 days before company B's, and she didn't join company A until B's approval, can she directly join company B without violating her status ? Is it compulsory that she MUST work for company A ? Is there a grace period to join company A in such cases ? 


2) If company A gets an RFE, B's approved and she starts working for B, will she then have to submit pay slips and H1B approval from company B to keep A's petition information as updated as possible ? 


3) I understand that the best course of action would be to say no to one of the companies, but I want her COS to be approved ASAP because if the original H1 petiion from Apr 2013 is acted upon and it results in a revocation, then her H1 won't be counted and she'll have to apply in cap subject category, in Apr 2015!! 


Please advise.