Yes, B can file for your GC.
No need to file H from B.
U can join B at any time with valid H or EAD. After GC approval you are expected to join B...
U can join any other company on a similar jopb after I140 approval and 485 RD6+ months, and respond to the RFE accordingly. In which case U need not join B at all.
After GC , all U need to do is maintain job in similar field to justify GC is the broadest assumption people make...