These aren’t instructions, it’s just a discussion topic, and nothing there tells you to install TWRP.
It’s one thing that should have worked, but didn’t, involving the data partition.
It’s possible.
I don’t think so. It’s not mentioned as a requirement at https://doc.e.foundation/q-upgrade-fp3, and I went from /e/ 0.12-p to 0.12-q without reverting to Fairphone OS.
/e/ gets the relevant firmware parts from Fairphone and includes them.
The e foundation and Fairphone have an official partnership in place.
You can check the date at Settings - About phone - Android version - Vendor security patch level … and they tend to mention when they imported from stock at Releases · e / os / releases · GitLab.