Ned is more formal, but both are equally accepted. Ner is the one usually used in speech.
I don’t know. Both are equally valid when I check the accepted translations.
Happened to me also in another sentence when I used "ner", and got a typo, saying "ned" was the correct answer. Is it possible to post a screen print in a comment? If so, I'll try redoing the quiz and see what I get
Yeah, you can upload it on e.g. Imgur and then post the link as a picture, but also report mistakes using the report button since we mods cannot go through all comment threads.