Used Brave for a bit during August and liked it (mainly their Discussions feature) but stopped using it since I always liked Startpage. I see a few mentions of Mojeek here, so I'll give it a shot
You can, but I don't know how/when they ask for it. When I changed my email on my main a couple months ago they asked for a number. When I changed the email of an alt recently they didn't, which was weird.
I like Skiff but I sent two emails with my Skiff email to my brother, but they never appeared to his email (checked spam and searched too). Only reached to him when I used my gmail address.
I'd consider that "downside" an "upside" (or whatever the word would be) since better quality, but I see what you mean