I would make a back up of their Windows. Force onto Linux, train them. Make all the icons obvious and look like Windows. Pin their most used applications. Let them run for a bit and see if they request Windows. You have the backup so they can always go back. I bet she never buys a computer.
I'm not going to lie to them, I think that would be wrong and try and tell them it's actually Windows, though if I were upfront and just made the icons look like Windows that would be ok. Even with the default Cinnamon icon styling, if I put Chrome on the desktop they'd figure it out, but even if I were to somehow objectively prove that OnlyOffice or LibreOffice or Calligra or <insert Linux office application here> are better than Microsoft Office, unless I can get the desktop version of Microsoft Office working on there I'm pretty confident that they won't go for it.
I recently tried to get my mom to use the PWA of Word online, and she said it was "worse" than the desktop version but refused to elaborate as to why.
I know I sound like I'm being harsh on my parents, but it's not like they're stupid or anything. They have spent the last 30+ years using Windows and Microsoft Office, I don't blame them for relying on the tools that they have spent the last three decades learning; I use command line tools that have existed longer than I have [1]. I do understand the stubbornness even if it does drive me a little crazy.
[1] Yes I know that GCC and Emacs and tar and grep have gotten updates since they've been released, but so has Windows.
This may or may not help, but have a look at ZorinOS. The latest release and it's announcement is targeted towards those wanting to jump ship from Windows -> Linux. They've tried to account for the adjustment with built-in tips/prompts and the UI looks closer to Windows also.
I've actually played with Zorin a bit in a virtual machine. It seemed cool enough, and I tried showing my parents screenshots of it to show them how Linux isn't that different than Windows, but they were unconvinced.
I managed to get Winboat working on my NixOS laptop, so it's possible that if I set that up they could deal with it.
I actually tell people to avoid Arch-based distros for beginners. They can break really easily with the rolling release. Something LTS would be better.
I never suggested you to lie to them, I suggested you set the UI so it’s familiar…because that’s really easy to do. Still inform them it’s Linux but make it useable like Windows.
Seems like you have it all figured out though, I think I’ve shared enough help good luck.
