If one never returns marriage certificate to the circuit court in Nevada after the ceremony is the marriage legal?

No, the marriage license has to be solemnized.

Within 90 days after the marriage ceremony, the person solemnizing the marriage must complete the marriage certificate issued with the marriage license and return it to the office of the issuing Recorder. Lack of witnesses does not render a marriage invalid. However, two witnesses can submit an affidavit that the marriage ceremony took place if the certificate is lost or destroyed and the person who solemnized the marriage is unavailable.