No, it isn't. ES is though. The only valid Scrabble 2-letter words starting with S are: SH, SI, SO and ST.
No. Fo is not an acceptable Scrabble word.
In my eyes yes RN is an acceptable scrabble word.
Yes. Flub is an acceptable Scrabble word.
Yes. Yo is an acceptable Scrabble word.
Yes. Da is an acceptable Scrabble word.
No. Eariest is not an acceptable Scrabble word.
No. Wop 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.
No. Excelsis is not an acceptable Scrabble word.
No. Viz is not an acceptable Scrabble word.
No. Cie is not an acceptable Scrabble word.
Abbreviations are not acceptable in Scrabble.
In my eyes yes RN is an acceptable scrabble word.
Yes. Jo is an acceptable Scrabble word.
Yes. Re is an acceptable Scrabble word.