It appears your case is under review after 221g. Contact the consulate for further guidance.

Selected as the best answer

You should indicate this country even if denied entry.

You must wait to become a USC before you can petition your parents.

It will take 14-15 years for sibling based sponsorship.

Yes you may postpone the ceremony again.

You should answer all questions truthfully and fully.

You can attempt to do so but likely to no avail.

The answer would be yes and it is a pending case.

Yes you can file the waiver and await approval before adjusting status.

Refer to the USCIS case processing page for better guidance.