As far as marriage is concerned, if they did get married somewhere, I would guess they likely got married either in Arizona or Nevada.
Now if they didnt get married in Las Vegas or Reno, could they gotten married in a rural area of Nevada? Did they have marriage chapels in rural Nevada or would they go to a justice of the peace to get married?
As far as id for a marriage license, Donnis could have used Donna, Marie, or Donna Marie as her first name. Another possibility is that for a surname, she could have used her mothers maiden name or her grandparents surname.
As for finding out information about Michael James Griffin, I would look at city directories around that time. You may be able to find out who his parents were if he had family living in that area.
If he did have parents living in that area, they are likely to be deceased. If youre able to find their obituaries from a newspaper on microfilm, the obituary may give you information about the parents next of kin and where they were from.