It depends on whether you're playing by rules that allow proper nouns. Nerf is a brand name, just as Vaseline, Kleenex and Band-Aid are.
No. Wop is not an acceptable Scrabble word.
No. Eariest is not an acceptable Scrabble word.
No. Fo is not an acceptable Scrabble word.
Yes, "nus" is an acceptable word in Scrabble.
Yes. Moot is an acceptable Scrabble word.
Abbreviations are not acceptable in Scrabble.
No. Excelsis is not an acceptable Scrabble word.
No. Cie is not an acceptable Scrabble word.
No. Viz is not an acceptable Scrabble word.
In my eyes yes RN is an acceptable scrabble word.
Yes. Jo is an acceptable Scrabble word.
Yes. Re is an acceptable Scrabble word.