After researching, I have thought long and hard about this and what I would do. So, I would probably try to narrow down when and where your grandma Donna was last known to be by talking to any of HER family (siblings/nieces/nephews, cousins), any friends of hers that you can track down, etc.
There is a possible very short (less than a year) marriage that may be your grandfather's. This will need solid verification. I would then track down the bride and/or any family she may have. To find out if there was any info regarding your grandma Donna that they heard. That marriage was September 1, 1967 and divorce was July, 1968.
If that is his marriage, we would now know that grandma Donna disappeared between the time your mom was born and August 31, 1967.
I would get every piece of paper & record you can find after your mom was born that could show any address/city they lived in so you will know where to report her missing if it comes to that. As well, that legal notice grandpa may have put in the newspaper to get a divorce would be good.
That is where I would go from here and then narrow down the timeline and place she was last KNOWN to be. Then I'd touch base with LE and see if they can add anything to the information you have. IF she isn't found, I would put together a very informative "poster" and start getting it on social media and get it shared. You never know who may have information, family rumors, etc.
I hope all this makes sense, I am having to rush but wanted to check back in