is H-1B extension possible with approved i-140 from old employer beyond 6 years?


i moved to new employer B in march 2016. i have not started my GC process with new employer B.

my old employer A filed my GC and also got approved I-140 in april 2015. Currently i am about to complete my 5 years stay. Now i planned to move to new employer C, so i am not interested to start my GC process with current employer B.

the below are my queries

  1. is that possible to get H-1B extension beyond six years based on my old employer A approved I-140 without starting my new GC process with current B /next new employer C?

  2. is that mandatory in my case i should have started my GC process before 365 days with current B/next new employer C when i complete my 6 years stay?

please help me with my above queries.

thank you.