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.
My sincere thanks to Mr Rajiv Ji and Mr Kunal Ji. I got my I-140 denial decision reversed in 7 months. I have this issue since 2012. I-140 originally files in 2007 and since then its been under initial review and received RFE/NOID In 2012. We approached the same attorney who was handling my I-140 and that didnt go well and ultimately it was denied. We approached Rajiv along with my employer and were in discussion for 20/30 minutes and said winning this case is fairly possible. Even I-140 petition wasnt supplied by previous attorney. Rajiv and Khanna briefed their arguement with me and asked me if i have any concerns so that they can add those in their arguement and discussed cons and pros and finally draft been finalized and sent appeal(I-290 B) to the AAO office. Our arguement was purely based on Labor copy and in 7 months appeal sustained and initial decision has been reversed and they approved I-140. We are now waiting for the approval copy so that have to go further and repair few more things. I am happy to say that i am going with Rajiv office to correct other things as well. WIth huge relief -Satish