Yes you may sign as parent guardian.

Not necessarily but he may be asked again at time of entry.

You may file for your child but your child is not eligible for AOS based on information provided.

Yes he should have his biometrics done in US for reentry permit.

Work with an attorney for respond to the RFE after thorough review and analysis.

This message indicates the I765 approval not the I485 approval.

No this is not a basis for denial of GC application.

Yes this should be acceptable for purposes of AOS filing.

Best to consult with a tax professional for guidance.

Your visitor visa should remain in effect based on information provided and should be used for future entry.