Iceraven for browsing, Ironfox for anything that screams “don’t open this”, Fennec for stored logins only and Cromite if I ever needed a Chromium browser.

1, Find out whether she needs any specific piece of software that is hard to replace. 2, Regarding distro, you should install her what you use, so it is easier for you to help. Ubuntu is well hated for a reason though (Canonical doing big tech things). I’d recommend either Linux Mint or Debian (if you are not a beginner in Linux and comfortable in the termunal) for both of you, but I’d wait until she passes her exams and may consider setting up a dual-boot on a spare SSD first, so she is able to try Linux and go back to Windows when needed. 3, Desktop environment: GNOME is considered best for ones coming from Apple and KDE from Windows. Both are resource hungry in my opinion, so I use MATE, which looks like GNOME 2 out of the box and uses a bit more resources than Xfce, well configurable though. Note that Ubuntu MATE is its flagship edition, many options are there out of the box (like MATE tweak).

Calyx seems to be the best option for you. https://calyxos.org/ Next time you buy a phone (and live in the EU), you may consider Volla. It’s degoogled Android by default and dual-bootable with an SD card, so yo can try other OSes while keeping your daily driver.

Thick (~1m) mudbrick walls. I lived in a house like that for 2 years, it felt cold inside in summer.