Under the current laws, if you change jobs after I-140 approval you keep your priority date, but you lose your right to H-1 extensions beyond what is given if your old employer revokes/withdraws the I-140.
Now answering your questions
Answer 1.No, you will get an H-1B extension for the time remaining and you can actually claim one year extra if your PERM was filed more than a year ago. But you will get only three years if the I-140 is approved. Not if it is pending.
Answer 2. I don't recommend it. I think you should get your I-140 premiums, get it approved and then leave if you want to.
Answer 3. I don't think that is a major issue, but do talk to your lawyers. Hence it makes sense in my view not to change until the I-140 is approved.
Note: This is a verbatim transcript of the referenced audio/video media delivered as oral communication, and, therefore, may not conform to written grammatical or syntactical form.
I would like to thank you all at the law offices, Mr. Khanna, Charu, Rena and all others involved who worked hard on my case. I was quiet nervous as my case was the first such complex cases being sent out (I was on OPT and H1 cap was reached), but I did not have an iota of doubt in the proficiency and efficiency of your office. I am glad I opted for the services of Law Offices of Rajiv Khanna.