Job change with in 180 days of I-140 approval

[T Bhanti]

My I-140 got approved on 03/07/2017, i got a new offer today and they are asking me to join by end of July 2017.
i want to join in a new company. lets say my current employer will not revoke or withdrawn my I-140 until Oct 2017 with the mutual agreement

1). Can i retain the same PD and use for multiple H1B extensions for the new employer?
2). Will it be an issue if they revoke my I-140 in Nov'2017 since I worked only 4 months with that employer (i think there is a rule to work 180 days with the employer after i-140 approval)
3). 180 days rule after i-140 approval- is this employer specific or not?

I would really appreciate if could answer asap? Thanks