It's Purim today, I'm drunk, got lots on my mind and feeling like typing it out here so please indulge me.
So to start, I have to ask, why is Crisis Core Reunion a thing? Oh one can definitely say it's a cash grab, the original game was beloved and remains a massive nostalgia trap for many, myself included. The magic of playing a game that subverted every expectation and led you to believe you could actually change the ending you knew was coming anyway (see the Remake trilogy parallels?) was a moment that everyone deserves to experience, even if it's less impactful now as then.
The devs stated, at the time, Crisis Core Reunion would be key to understanding the events of the trilogy, but this can't be the answer either because it came out after Remake. Or can it? In fact, I posit that the plan was always to re-introduce Crisis Core ahead of Rebirth because it's key to understanding Aerith.
This is undoubtedly Aerith's time to shine. In OG it's the period of the game where her past is fully explained, her mission is elucidated, the attempts to chase her and ultimately stop her fail and she dies symbolically handing over the key to save the world. Rebirth is much the same along these lines - though one could certainly argue Remake and Rebirth have removed a lot of scenes from her and added a lot more to Tifa, nevertheless it's still Aerith's time to shine most of all. She has much to accomplish, and her role is pushed to make the loss hit even harder, the song she sings is as much to the viewer as to anyone.
But where does Crisis Core come into this? One line stands out to me from Aerith to Zack "I have twenty-three tiny wishes, but you probably won't remember them all, so I put them all together into one...I'd like to spend more time with you."
Aerith's one wish is to spend more time with Zack. She could ask for anything here, but that's what she wanted and, indeed, that's what we're shown she's attempting to do in Remake and especially Rebirth. Zack isn't around, it's true, but wait, a guy falls into her life in exactly the same way, with the same eyes like the sky and the same sword to boot. In a world where reincarnation is common place and a desperately lonely girl has sat and waited some five years yearning every day, this is all too much a coincidence, all too great a chance not to be seized - to test the waters.
So she makes her wish a reality by spending time with Cloud, treating him exactly the same way she did with Zack and expecting similar responses, relating to him on a level that he's just about able to play along with - thanks to his own Jenova-jumbled personality and memories - but completely unable to reciprocate due to his deep personal feelings for Tifa. It's all an act, no less a play than Loveless, but without either party fully able to be themselves. Cloud completely wrapped up in his SOLDIER fantasy, indulged fully by the bodyguard status Aerith gives him, and Aerith so desperate to just have her Zack back that she's genuinely believing, to a degree, that he is in Cloud.
The devs wanted us to see that Aerith's heart yearns for Zack, that they never had any proper closure and indeed are broken up only physically, but not emotionally. They also wanted to provide this context so scenes like the Sector 5 Dream Date, a complete recreation of the very scene from Crisis Core - right down to the NPCs, who now instead of complimenting the couple are negative towards them - make narrative sense regardless of your affinity.
It also comes to redeem Aerith. Ignoring this context, as Clerith's would have us do, renders her little more than a floosy who casually thirsts for a man she just met but has been the target of the affections of her friend for over a decade. Further, how can one argue she's over Zack when all the evidence proves otherwise? The love letters, the pink dress to fulfil the promise (evidently, the promise to wait for each other was the last discourse they have) there's just too much evidence that speaks otherwise.
Anyway, I guess the point here is that if you find yourself ignoring some part of the compilation to make your ship work, then it's not actually working at all.