Urgent: Urgent: Multiple H1B Visa Transfer - Change of Employer Need help

[usertest1919]

I am on H1B visa. I have signed an offer by a company A and they initiated the H1B visa transfer in premium processing, which is in process and I also have received Specialty Occupation RFE on my case.

Now I have an offer from company B, which is so good that I couldn't reject. Now I am planning to sign company B offer. Now Company B is ready to sponsor H1B transfer. I wanted to know ,
1.) am I eligible to start working as soon on arrival of receipt wit company B , as they dont want to wait till approval of my H1B.

2.) Please let me know if I would start working on arrival receipt with Company B, and meanwhile if my RFE gets resolved and gets an approval from company A for my H1B case, can I switch my employer from Company B to company A. (Both Company A and B has filed my H1B change of employer H1B application from my current employer.)

3.) If I pursue 2nd option, Do I need to file H1B application again to move from company B to Company A.

Please share your experience and thoughts as I just want to take up one of the opportunity by playing safe game. Thanks for all your help in advance.