I am no expert, but i can speak from my experience. I was in the same situation as you, a year back.
1) I did not transfer H1 when i switched jobs. Just provided the EAD as work autorization doc for the new job. From my experience, there are no issues with switching on EAD. I dont think you should worry about I140 revoke. Even if your current employer does that, since it is past 6 months after the I140 was approved, it should not impact you. I know there were few cases on trackitt where they got a NOID, but they were able to appeal it and get corrected.
2) refer 1
3) I waited for rfe. my new job title was completely different. the roles and responsibilities were kind of same but not similar. this was not an issue at all. i responded to rfe and the case was approved.
4) dont know
5) It appeared fairly safe to take a better opportunity.