I guess my parents are/were of a cautious enough sort that, for them, the trouble of navigating the internet turned more into narrower use rather than installing tons of malicious extensions via questionable websites. (I do remember once finding an .exe file downloaded from the website of a magazine to which my father subscribed. Didn't work in Ubuntu, I guess. Probably also not malicious in that case, but I can see how people could end up with crapware.)
As for other issues with basic use, my mother is even older now, and even things that used to be pre-arranged and familiar have become more difficult. But I don't think the OS matters much for that any more, on a grand scale. Touchscreens might help to an extent (and also avoid upside-down mouse issues), and obviously things would need to be set up so that there aren't any unnecessary hurdles. But beyond that it's learned routines rather than any kind of a generalized understanding anyway. She does know how to usually get rid of things by clicking 'x' though.
Did you find something that worked out for your grandma in the end?
(edit: You edited your reply before I finished mine, so I replied to the earlier version. I see the problem with accidentally triggering things. I guess it might be the fairly cautious nature and the limited use patterns that might have saved my parents from them.)