If you missed the due date, you are out of luck and you may wish to refile the I-751 instead. See a lawyer.

If the marriage is recognized in Nigeria, then the U.S. will also recognize it.

She can stay in the U.S. until there is a decision on the adjustment application.

Getting married will not put him in danger. It may be difficult to get him approved for the U visa as a derivative due to his criminal and...

Correct, you must depart and you can return to the U.S. with a new H1b.

I would guess about six months, but you could check the processing times at www.uscis.gov.

It could. But generally traffic tickets will not prevent one from naturalizing.

Take a look at the form instructions and it will tell you what evidence you must submit.

Here are a few: you can adjust status even if you overstay if you are adjusting as a spouse of a U.S. citizen, as a U visa beneficiary [crime...

It depends on the specifics. If they do not report to you, it seems you do not manage them. If you can show that you do in fact manage them, then...