I don't think Aerith is this selfish, manipulative character who only cares about dating Cloud.
If she only cared about getting with Cloud then the game would be vastly different. She wouldn't outright tell him she still loves Zack, she wouldn't question her feelings so openly before the end and, ultimately, it would all be an open betrayal of her friendship with Tifa who she knows has loved Cloud since they were both kids. It
demands a different interpretation.
For me, this is simply yet another character assassination, a flanderisation of the characters so square pegs can be fit in round holes. Aerith loves Cloud, simple, case closed, we're told. But such a statement, even if true, lacks all the nuance and context of what has been shown.
Picture the scene: one day a handsome man falls into Aerith's church. He has eyes the color of the sky, she's scared but his eyes smile at her, they have an instant spark and match each others energy. It turns into a deep romance and attraction, calling each other during battles, helping and supporting each other through life's problems, growing together. Then, fate cruelly takes this man out of her life. For nearly 5 years she continues to wait for him, send him love letters, wear the pink dress and ribbon, sell the flowers, miss him more each day until...
A handsome man with eyes the color of the sky falls into her church. Can you imagine how Aerith would feel? Leaving aside what the existence of the Lifestream would bring to it, the confusion, the hopefulness, the desperation to end her loneliness. All of this comes bubbling to the forefront.
In the two short weeks they know each other, they become good friends. But for Aerith that connection with Zack cannot be escaped. It's understandable she would slip into treating Cloud like he's Zack - indeed she admits to doing this - and instead of a malicious attempt to get between CT her actions are, at worst, an attempt to recapture a lost love that was doomed to failure from the beginning.
I don't think she's selfish, manipulative or backstabbing at all. Ironically it's the Cleriths who are left with this view of her because they remove the context of her love for, and missing of, Zack.
It's all on the devs now. Part 3 needs to complete the circle, show us the Reunion (as Zack says, so to as two worlds drift apart they will come back together) and give these characters, all of them, the happy ending they all richly deserve.