I've continued to pursue death dates and obituaries for everyone in my list with a custom ID of D (dead or probably dead based on their date of birth). It's been slow going as I've been tracking down local libraries and contacting them to see if they offer an obituary lookup service.
In addition, I've added a few requests to Random Acts of Genealogical Kindness. RAOGK volunteer Raymond in the Dallas, Texas area responded almost immediately with an obituary for Marie.
Marie already had a memorial on Find A Grave; however, it was under Marie. Most of the records I have found for her were under Corbitt. Therefore, I added a second memorial and cross-referenced it to the first momorial. See 6734531 and 67581124 on Find A Grave.
Raymond also told me that he couldn't find an obituary for Marie's husband:
Donald Richard Knopp
b. 12 Jun 1926 Minneapolis, MN
d. 18 Mar 2005 Irving, TX
Marie is buried in a national cemetery. Therefore, I'm assuming that Donald was also buried in the same cemetery. I've requested a photo of Marie's gravesite in the hopes that the F.A.G. volunteer will notice and take a photograph of Donald's gravesite too. We'll see.
In the meantime, I'm still in hot pursuit of the right library. When I'm successful, I usually end up making a donation to the library when they don't have a fee for the service. I figure if I have to go back to them a little bit of cash will grease my way into their good graces. After all, many times, librarians are looking up obits for me in their spare time...time that they could use for their own pursuits.