Best to check USCIS processing page for guidance.

It can take 18-24 months for I751 to be processed.

Yes this isn’t a problem with the pending I130.

Best to contact center to inquire about process.

Yes this is completely normal and nothing to be worried about.

This will not create a denial for naturalization purposes.

Yes best to wait 90 days before applying.

Yes this should be acceptable but refer to USCIS form instructions.

Yes he may do so but the child support issue should be resolved.

Your sponsorship remains until the applicant becomes a USC.