Interesting theory, and I do think they’ll bring in Jafar as S4 villain somehow.
But I think Zelena’s curse WILL work, because i’ve just been thinking of what she’s going as a variation in technique on what Jafar and Amara did. Zelena needs courage, a brain and a heart. Cyrus (and Will) had courage/bravado, his older brother seemed to be a thinker, and his younger brother seemed to be a sweetheart. Same 3 ingredients, just in different vessels to the ones Zelena is collecting. On WL it just so happened that the 3 ingredients existed in the form of Amara’s sons, and her purpose was to be able to reunite her family, so she could conveniently reunite them and use their reunion as a way to cast the spell to free them because she had courage, brain and heart from each of them.