Yes you may file the I130 while he is visiting.

If he is over 18, your son can apply on his own & can provide valid state ID for jurisdiction purposes.

The affidavit of support is not relevant for citizenship purposes.

You can still file despite unknown re: relocation of stepchild.

Your question is unclear. Contact USCIS or an attorney for review of your case.

It depends on when the overstay happened. Consult with an attorney for guidance.

If there was weapons training in the ROTC, then answer accordingly.

Domestic travel with valid ID should not be a problem.

The marital status should not impact the case given nature of filing.

You can obtain the I485 through the USCIS webpage.