First off, thanks! :D These fic are a kind of experiment of sorts for me. I was shooting for a dichotomy between Cloud and Kadaj--they started from opposite points--one originally whole and one originally incomplete, and that meant one would be struggling to stay complete, while the other reached for completion, and that made all the difference in the end results: Cloud shattered and lost himself while the fragment became Kadaj.
One thing I had in mind, with the Zack aspect at the end, was having Zack in a way perform a kind of mitzvot, to help with the Tikkun ha-Olam ("The Restoration of the World"--basically, recovery of the divine "sparks" from the kellipah (husks) of the sephira destroyed by the shevirat ha-kelim. Or rather, Zack is, in his way, the tikkun--rectification, good; the presence of Divine light, in opposition to the kilkul--damage, evil; force strengthened by the absence of Divine light.)
For the Tikkun, people have to find "sparks" which are relevant to their life, and basically reassemble the vessels in a way that was superior to the first, imperfect version. Which...Zack did. In a way. Cloud as he ended up was one capable--though it wasn't easy at all--to not completely lose himself or shatter again when he had to face Sephiroth taking him over again.