A parent may sign for their minor child.

Best to consult with your employers counsel for guidance.

USCIS will reuse previously captured biometrics.

No not at all does this rise to the level of suing USCIS.

Not after he has his citizenship and unless you have actual proof of fraud.

This would be best served and assisted with NVC phone support or online support.

He may be able to with a cosignor. Consult with an attorney for guidance.

It may take a few months before decision is made.

Best to obtain a I551 stamp to ensure reentry.

The I751 may still be filed late and with a waiver.