Change employer without working for H1B sponsoring employer

I am currently in US on L1B from Employer A (petition and I-94) valid till Feb 2021 and would have another 2 years to max out (Feb 2023).
My current employer is not supporting to file H1B, so I want to apply for H1B consular processing through another employer B this year, so that I can initiate PERM/I140 and secure an early priority date.
I understand that with consular processing approval, USCIS would give me I797-B (without I-94), which means I can continue to work for Employer B, till I re-enter US with H1B visa stamped on passport.

Assuming my H1B would be picked & approved in 2020 with 2-3 years validity, I have below few queries:
1. Can my H1B (if approved for Employer B) be transferred to Employer A, while I continue to work for Employer A?
2. How long can I wait (and continue working for Employer A on L1B), before going for H1B stamping and start working for Employer B (assuming Employer B would not revoke my H1B anytime soon).
3. Can I change employer i.e., work for Employer C immediately after I get H1B stamping done?

Thanks for your feedback.