Working for company A who did concurrent filing for my I-140 and I-485 in EB1A category on Dec 5, 2008. 180 day will be on June 5. The cases are still pending at NSC. Got my EAD and AP approved in Feb '09. Currently on H1B until Sept '09.

Want to leave the current employer ASAP but before I accept any other employment I want to take a month off to finish my Ph.d. Fairly certain my employer will not withdraw their I-140 after I leave. Not sure if I-140 will be approved by the date I resign.
1) When is a date after which I can resign from my current employer while keeping GC process alive?
2) If my I-140 is not approved by the time I resign, can I still stay in the country to finish my ph.d?
3) When will I-140 approved in your opinion?
4) Should I use H1B to return if I travel?