No, a pending I-130 does not extend one's lawful status in the U.S. so no, this is not a way to try to stay in the U.S.

Yes, if your application for employment authorization is approved, you are allowed to start working in the U.S.

While generally it is not recommended, I have had many clients do it without any problem, and naturalized without any issue so it's at your peril.

It seems to me that you cannot enter the U.S. on an H1b if you are not entering in order to work for the H1b petitioner.

You cannot circumvent the two year home residency rule, so you should plan to return to your country for two years.

If you are required to renounce your U.S. citizenship in order to attend medical school, then yes, you could lose your U.S. citizenship.

Yes, anyone filing an I-485 can file an I-765 together with the I-485, including someone filing an I-485 as a derivative.

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Sure, once he has been out of the U.S. for ten years, he may be eligible to file an application to return.

Yes, in my opinion there is a substantial chance the J1 could be denied because you have an immigrant visa filed seven years ago, which may be...

File for an extension of status, so that they can make it to the graduation ceremony, and proof they are returning promptly after that ceremony.