H1 transfer with 3 months to I94 expiry


I have an approved I140 from my current employer A
My 6years H1 max out by Mar2019.
My current i94 is valid till dec31-2017
Employer A has not filed my extension yet hopefully they will file in this month I assume.


Now with just 3 months left in my current I94, can employer B file my visa transfer ?

If B files transfer and it gets approved by November , how soon should employer B start filing my PERM n I 140?

What happens to the extension filed by A?

If my PD should be ported to employer B , what documents or evidences do I need to obtain from employer A before joining B

Thanks in advance