I have an approved 1-140 from employer A. My 3 years H1-B extension beyond 6 with employer A years is underway (Current H1 expires on Sep 30th). Now I have another offer from employer B.

1) Would the existing extension request from employer A be any negative influence on the transfer request from employer B both happening at the same time?

2) Should I wait for the H1-B transfer to employer B be fully approved before I let employer A know to avoid any complications arising due to employer A revoking the I-140. Or can I leave employer A immediately after I have the receipt for visa transfer?

3) Employer A has not provided me a copy of my I-140 approval notice, would a copy be required for employer B to transfer the H1B or just having the I-140 receipt notice will suffice?