Yes you may attend school with a pending application for I485.

Yes this can occur in the course of adjudication.

Yes this isn’t an issue and can be addressed at interview.

It should indicate the date since you were a LPR.

Yes this should not present any issues.

Yes you should be a USC to petition your parents on a tourist visa. Their intention cannot be to enter and adjust status.

It is best to have the petitioner attend the interview.

Yes you may upload to the site now.

Best to him consult with an attorney to discuss his options. Good luck!

You can appeal the decision if so.