Wow it's been a while since I stuck my nose in here; just a couple of things...
ZaCloud is not some alternate being. It's Cloud using the mannerisms of this successful, impressive guy he wanted to be like.
Aye, but he's not using Zack's mannerisms
consciously. I.e., he doesn't think, "Wow, Zack was a cool guy, and Zack would've done this; therefore, I'll do the same." He really thought "This is who I am, and this is how I'll react." There's a huge difference between the two.
Furthermore, Aeris actually states several times she isn't falling for Zack in Cloud. ... Since Aeris knows Zack the best, she knows what parts are his and what to separate.
She mentions that? Several times? I must've missed that, could you please refresh my memory? I apologize if I've missed any quotes/examples since that post.
You appear to be speaking pre-Death-Of-Aerith. Sure, Aerith knew Zack the best, but there's no way she could've known, or suspected, that Cloud's behavior was modeled on Zack's - at least, not before she died. Therefore, she couldn't have known what parts were his and what to separate, simply because she didn't know the original Cloud.
... but she makes it pretty clear she doesn't want a replica of her ex-boyfriend.
In the light of Crisis Core, I'd say it was pretty obvious she first latched onto Cloud because of Zack, and that she wasn't over him. There's no way the creators of Crisis Core would've bothered to put in those little ties between Zack and Aerith just for laughs. When you play Crisis Core, and then play Final Fantasy VII again, keep in mind a few things.
1. Up until a few months ago, Aerith had written 80+ letters to Zack, over a period of over four years.
2. Aerith gives up on Zack ever returning her letters - she probably assumes he's hiding from someone and/or found another girl.
3. She feels a Disturbance in the Force, and possibly attributes it to something terrible happening to Zack.
4. Just a few weeks later, Cloud falls through the roof just like Zack does, into Aerith's life.
5. She uses a line on Cloud that Zack used on her.
6. Aerith wears a pink ribbon - possibly the same one Zack had bought her. She also continues to wear pink - something Zack asked her to do for the next date. Both her ribbon and her dress have strong ties to Zack.
This girl acts like some very unwell Zack fangirls I know.
It doesn't look to me like she doesn't want a replica of her ex-boyfriend. It looks to me like she wants her ex-boyfriend, period.
... as Aeris isn't a delusional person..
If she did realize that Zack died at the end of Crisis Core, she's pretty good at deluding herself, though...
It never struck me that she was falling for Cloud because of the Zackisms, just that the similarities finally caused her to deal with something she really didn't before (because Aeris has no friends, and no one to talk to lol).
Try playing Crisis Core, then approaching Final Fantasy VII without any preconceived notions from the first time you played it. I didn't think she was attracted to the Zack in Cloud the first time I played the game - because I didn't know. But Crisis Core puts a whole new spin on things...
Oh, and about the flowers... If flowers represent Aerith in Final Fantasy VII (which I think they do, and note I said "Final Fantasy VII," not "Final Fantasy Series"), can someone, preferably Clerith, please explain the symbolism in the following?
1. Tifa and Cloud lying face to face in the flowers in AC/ACC.
2. Flowers growing over Zack's grave.
3. Someone - presumably Cloud - putting Zack's Buster Sword in Aerith's church, near her flowers.
Sorry, Tenten. 
