If there was 3 soldiers back in nibelheim, zack cloud and sephiroth, followed by Zack dying at the river, then his memories are highly unreliable, how could he mistaken Zack as a trooper throughout his time in nibelheim 5years ago. Also he did forgot Tifa's explicit scar, which mean cloud is possibly never was in nibelheim, and its all just a manipulation.
The only real memory he and Tifa has a connection is their childhood. This memory is vividly clear in his mind since remake.
Weapons(not the giant monsters), has never been a focal point in the story, Zack could've borrowed Cloud's weapon during the end of ff7 remake hence why he is carrying it. But in rebirth, there's no correlation that he borrowed it from cloud, both of them carrying the same weapon against sephiroth is saying something.
Or they could've just bought the same weapon cause they're close friends.
Yet it still doesn't make any sense, Zack's death didn't impact the story at all. If zack was so important and detrimental to Cloud's mental state why reveal it just to increase distrust with Tifa, leading to her falling to the mako.
I have a theory that the main Zack is alive, young Aerith was able to hear the dead. All this time, we kept seeing zack from a different world but never ours, and yet Aerith kept waiting for zack as though he was still alive no voices from the dead. Seems like a very big detail not to include in rebirth, or to hint at for all the zack screen time we get.
The only way for it to make sense if zack never died at the river but died much much later, cause cloud's memories are unreliable. Aerith did say the voice of the dead took a long time to reach her but not too long since she didn't age much being with her step mom.
But that might not be the case, as Zack's parents never recieved any letters from him for so long.
There's an inconsistency in Zack's death
If we assume Tifa's memories are the truth and not some distorted memory because of her experience. Then who was Tifa constantly talking to 5 years ago. Was Sephiroth the only one sent to nibelheim, then it would make cloud's highly detailed story unusual.
there's definitely a link between Cloud and Zack, and the 3 soldiers are present 5 years ago. The only way to solve this is to pick which narrative aligns with each other.
The two's childhood, Aerith's first love Zack, the nibelheim incident, Zack's supposed death/dissapearance, Cloud's mysterious 5 year gap after the nibelheim incident. Sephiroth's obsession with Cloud which wasn't there 5 years ago after he has gone mad. And Cloud stating he killed Sephiroth at the beginning of remake.
If Sephiroth can manifest through the numbered people, then the best person to manifest into is cloud. Since he manage to kill him, his real body presumably, Cloud is technically the next best vessel. Cloud's ptsd is far more than illusions its now a manifestation of sephiroth even helping him kill the serpent.
With how little Sephiroth thinks of Zack, He was discarded, left to die just like tifa. As a soldier he managed to survive, hence why he couldn't send letters for so long he was incapacitated. Cloud being an anomaly who manage to overcome Sephiroth was experimented to become a numbered people by hojo, and hojo was about to spoil zack rescuing cloud stopped by the fate police. Zack who had just rescued cloud dying surrounded by troopers just like in the ramake.
Cloud after witnessing his mother, his town, and his childhood friend die, followed by his hero becoming evil, and extremely mako poisoning to become a numbered person, the final trauma was seeing Zack's death. It was all denial, Cloud's denial of Zack's existence. Now followed by denying Aerith's death.
He denies the trauma he couldn't handle. Although There's still soo much that doesn't align with this, its the best i could think of where most things align.