Wait so did it really matter what side Zuko was on it was completely Win: Win
If he sided with FN then he'd come back a hero , the FN would win the war?? his life would return to normal and he'd eventually become the Firelord
but if he stayed with EK, then the Azula would be out of the picture (sorry she's not bad enough to take Aang,Katara and Zuko) he'd gain the trust of Iroh, Gaang and EK ...plus the Black Sun envasion would go forward which would end the FN ...and after Ozai dies then who is the only eligiable person (with the trust of EK and WT) that could take his place....duh Zuko