Salam,
I really can't understand why did the twelver ignore ismael ibn jaafar, most of twelver say jaafar as sidiq choose Musa kadhim ... Ok? so how about this :
https://en.wikipedia.org/wiki/Abdullah_al-Aftah ( if musa kadhim was choosen why the majority went to abdullah al aftah ? )
Also let's say there is a king , who have 2 sons, the eldest died before his father BUT he had a son. So who will be the next king? in 95% it's the son of the eldest son, not the little b