Proton serves privacy, not anonymity. They will not collect, harvest, analyse or sell your data. If you however use their services for illegal things they will forward whatever - usually little - unencrypted information they have about you.

You can always compile from scratch, compare the checksums or use the version you compiled. In projects this large people usually do this, and there’s a certain level of trust that these checks have been performed.

Mint looks quite similar, and if she asks “Microsoft applied an update and now the start menu looks different.”

If all she does is browse the internet and read emails, she’ll never know the difference. You could even set up the splash screen to display the Windows logo or just disable it all together.

Wikipedia itself is pretty great, the company and its marketing behind it, not so much.
https://www.dailydot.com/debug/wikipedia-endownemnt-fundraising/

I agree with everything but using Google sheets. It’s neither free nor open source. You don’t pay with money but with your privacy. Libre office is just as good as a desktop application and is actually FOSS. If you absolutely need the cloud storage, get a provider you can trust, buy the space and sync your files online, after editing locally.