Oedipus complex explains Cloud's love for Aerithit was Zacks memories and Person in Clouds body that was causing mixed signals to fire.
There are hints to this in game, throughout the compilation, and out of game.
In the first Nibelheim flashback Cloud's mother tells him to date an older girl. One that will watch after him.
- If you read the biographies in the game manual, Aerith is a year older than Cloud and Tifa is a year younger than Cloud
In Advent Children, Aerith is confused as to why everyone keeps calling her "mother".
In an interview with the original developers they are asked why the choice to not be able to revive Aerith was made. The development team says that Hironobu Sakaguchi's mother had just recently passed and that they wanted to show that death is permanent but that those who have passed still "live on" and still very much effect those who are still living. Each developer had their own take on it as well.
Stress induced schizophrenia explains Cloud's dissociative memories.
Cloud doesn't technically have any memories that aren't capable of being his own. In essence, Cloud doesn't falsify memories of times when he wasn't in Zack's physical presence like memories from Zack's childhood. Cloud's childhood is Cloud's childhood.
While Cloud's love for Tifa could indeed be considered romantic love , he is hesitant to commit because of intense feelings of loss and powerlessness.
- Loss of his father then later his mother.
- His inability to protect Tifa on Mt. Nibelheim when he was a child and his subsequent berating by Tifa's father.
- His inability to protect Aerith.
- He couldn't stop the Midgar plate from falling.
- His inability to make it to SOLDIER.
- His feelings of helplessness when his town burned down.
- The loss of his best friend, who he felt was more worthy than him.
Some of the above points cause Cloud's disorder, some reinforce it during the events of VII until his "realization".
Before the events of VII, the only true act of heroism was when he attacked a weakened Sephiroth at the reactor on Mt. Nibelheim. Even then it took a great deal of the above for him to do it and it was mainly out of an emotional desperation. He had been pushed so far, he just couldn't stand idly by.
If Zack never existed, Cloud would still have plenty of psychological motivation to explain his love for Aerith and his less than reliable memory.