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 cannot thank enough the Law Offices of Mr. Khanna for getting me and my wife our L1-A and L2 approvals. A special thank you to Mr. Jagbir, who assisted us, guided us and was there for us the entire time. He was very patient, helpful and informative, and Mr. Khanna was always available to answer my never ending questions. I can confidently say that it is because of their hard work and dedication to this case that we got our approval. We had a very complicated case whereby our LI-A had already expired, and the extension was denied. Mr. Jagbir presented our case with updated information and new evidence, and after he responded to the RFE with a 700 page file, our case was approved. Thank you so very much Mr. Khanna, Mr. Jagbir and Ms. Anna, we will forever be indebted to you.