that, and perhaps an element of "I'm gonna be Zack so that he won't be dead and leave me here alone" - not that any of it would have been conscious decision.

I guess, in this fic, that's what Cloud does. Because of Nibelheim his original idea of like Soldier = like Sephiroth (the whole reason the plan to go to Midgar entered his mind in the first place) had to get a rewrite. So, like Soldier = like Zack. He needs to be strong. Soldiers are strong. Zack is strong. So, he'll act like/be Zack.