The inner voice is Cloud's own. His true self, buried deep down inside.

Cloud's not having hallucinations, he's merely in a deep deep state of denial. The trauma he endured with Zack caused a deep kinship between the two of them, and Cloud no doubt felt like he failed Zack (when Zack died on the way to Midgar) along with himself, Tifa, his mother, and everyone else in Nibelhiem. Zack was everything Cloud wanted to be, and his guilty, scared, and messed up mind created this alternate truth, using Zack's past (what he knew of it) and his own in a sort of hybrid story.

He didnt share memories with Zack, that's clear. If he did, he would have remembered Aerith. All the stuff he knew of Shin Ra he learned during his guard days.

It wasnt for attention either, but it may have been partly because he felt like he needed to live up to all the boasting he did in front of Tifa, and he needed her acceptance.