That's great a 90 yo is still going strong and not about to let incontinence slow her down!
I don't think snap on's are any good for active folks. They just can't be secured well, so are more suitable for non ambulatory people.
I agree a pin on pre-fold is complicated.
I would pick a Velcro on diaper, such as Kins #10500 since the fit is customized and she can get the diaper as snug or loose as she prefers. And they are easily put on standing up, holding the diaper against the wall with your behind while you pull the front up thru your legs and then grab the rear wings with Velcro tabs and smooth across the front belly and you are done.
My second choice would be a pull on such as Kins #20700 which depends on elastic waist tension to hold it up....could be a problem for slipping down as it gets wet and heavy. That's why I say second choice. Some like them for simplicity of just pulling them on and if they can put up with the waist tension and they don't droop or sag too much, they can be ideal.
I think the more positive holding power of the Velcro is needed for diapers worn by an active person who is going to get the diapers wet in the course of the day and activities. The Velcro on will stay put, just as a pin on diaper would, resisting sag or droop as it gets wet.
She will need several considering drying could be a day, more or less on the drying rack or line....depending too on the heat and humidity in the house.
Best stuffers may be low cost flat baby diapers or baby prefolds. Both will dry pretty readily. they will fit nicely within the Kins diaper, either the Velcro on or the pull on.
If she wants a larger stuffer diaper, than get the squares. They can be folded in thirds and inserted. They will dry well as they will be opened up flat to dry. Granite Smith which you mention, I believe offers gauze squares in two ply and four ply. I think the two ply will be heavy enough and will dry well. When folded into thirds, that adds 6 plies of cotton to the center of the diaper. A second one would add 6 more plies, same as one of the 4 ply construction but the two separate diapers will dry faster than the one heavier one.
For plastic pants, my first pick is Kins #20300V which is the mid weight vinyl, soft and stands up well, along with comfortable Lycra leg and waist bands. They fit nicely over a diaper, a better fit in my opinion than the Comco pants.
Non plastic pants can be supremely comfortable. Kins 10300 is a great nylon waterproof pant with the Lycra bands.
But take a look at Changing Times Diaper Co (
www.changingtimesdiaperco.com) for waterproof pants with adult prints and colors. They also have juvenile prints, of course, but the adult prints are really nice and you may see something Grandma will really like. Made by Bumpkins they are high quality like Kins and feature the very comfortable Lycra bands. They are cut properly to fit over cloth diapers.
I'm sure you'll get some more input and please let us know the results and hopefully the sucess of your getting Grandmother set up for cloth diapers.