Phoenix-downing this thread because I am confused/curious.
From what I understood of the Cloud/Tifa/Zack memory arc/explanation in the game, I gathered that Cloud's insecurity and shame, as well as confusion post-Mako-poisoning, created his persona at the beginning of the game. I don't believe Tifa's personal memories had anything to do with forming Cloud's persona, because they're used as a control, or an identifier, to let Cloud know he existed prior to Nibelheim. The way I saw it, Tifa doubted her own memories because Cloud doubted his, not particularly because her memories were used to create nouveau-Cloud. More like, Cloud's memories of Tifa were used to do so. Zack, on the other hand, Jenova would have access to.
I don't interpret Cloud as being too mentally weak to withstand Mako infusion; I interpret that he is stronger than the others, and therefore not compatible with Shinra's desire to create physically Elite and yet mentally weak and easily controlled individuals. Jenova would surely reject them on that basis alone. Did Hojo's experimentation take place within Shinra parameters?