I've always thought
Joss as a nn for
Jocelyn. Even if you click on it it says
Joss is a short form of
Jocelyn. I'd go with your suggestions of
Joss and Jossy / Jossie or even Miss
Claire's spelling Jocie as nn for
Jocelyn.
Jojo could be but it requires changing the pronounciation of the 'o'.
Lyn could be too. You caould also try asking this on the name opinions board for more responses.
How is it that
Beth is an "official" nn of
Elizabeth? What about
Beth for
Bethany? How exactly do you have official nn? Is it a case of
everyone knows that this nn is for this name?
My 45 PPs - names in profile