Changing employer and INDIA travel

[Lolliguy]

My PD is Dec 16th -2013- EB2

Applied for I-485,I-765,I-131 on Nov -2nd RD for me and my wife with down grade EB3 I-140 (New) 

I-140 Approved in PP on 06/10

Not received EAD and Advance payroll yet .

1) can i change the employer with just I-140 EB3 approval on H-1B what are the implecations?

2) Can i change once i get EAD in hand ? Any dependency with my current employer (if he withdraws I-485 etc)?

3) Can i travel india and come back with I-131 adv payroll doc?

4) If in case I-485 is denied for some reason can i shift to H-1B ASAP? or Refiling options and can i be in status at that time .

5) I have two I-140s one in EB2 and one in EB3 , which would be i be considered if one goes faster ? can i request USICIS to consider.

6) Do i need to apply for NIE if i travel on EAD to return back to US ?