on GrapheneOS you do not need the legacy "manage all files" permission. GrapheneOS has a specific "obb" permission for installing apps, thats it and even that is legacy
Disclaimer I am on Fedora Kinoite with soon Plasma 6 too.
Staying in an older Fedora Kinoite version will spare you from the breaking changes. Like currently 38 instead of 39. I would use ublue kinoite-main. You can disable animations, baloo etc. and have a very minimal experience. (Kinoite is way lighter than Fedora KDE and doesnt even include Gwenview, Okular or Kate)
Have a look at EndlessOS. Easy Desktop, immutable Debian base afaik. VanillaOS also has a Debian variant.
Immutable stable Distros are really needed. But I think Fedoras rpm-ostree is currently better, because it uses a git-like approach. I also think it is overcomplex and moves too slowly, so I imagine something may surpass it.
Automatic upgrades like traditional distros are not enough (they only annoy users but dont really apply them), you need really automatic ones.
Ublue has ublue-update on some editions, which is really nice. Fedora wants to implement some half baked solution I guess. If your Grandparents are always at home and on power that is no problem though (metered networks, low battery, AC connected).
For Fedora with NVIDIA I can just recommend ublue -nvidia images which work out of the box.
Linux Mint DE gave you these problems because of XOrg, in a year or so their Desktop will be ready for Wayland.
No its not the best of GNOME with a traditional Panel etc. Its a far deviation, dasht-to-panel is way better or use ZorinOS which is kinda strange but partly FOSS I dont know really. Their GNOME customizations should work everywhere.
No. Appimages are selfcontained and thus useful for archiving software or carrying it around in random ways. Flatpak could do this too but not as easy.
Also maaany packages direct to issuetrackers of projects not supporting that flatpak.
If someone knows where that flathub metadata is stored I would love to know, as the manifest is not it. I would like to fix those to link to their own bugtrackers
Yes the entire account system actually works on GNOME while I also find the KDE alternatives way worse.
Still, Zorin is modded GNOME so even better.