I do think that Zack has moved through time without realising it, in the singularity. As you said, if he had seen the plate missing, then he would have worried. However, we can summit that, if Shinra did indeed kill Barret, Tifa and probably someone else from Avalanche, would they make the pillar fall? Probably not. Besides, as I said, it's once the FFVII timeline starts that Zack and Aerith cannot be alive at the same time, ergo December. Whether the two timelines are merged or not remains to be seen (probably not).
Well, he hasn't visibly seen Aerith for a long time (he's practicing in front of the church), so chronologically, that's the only time when it fits. Whether it's right after or in December is unknown, but if in the OG Aerith has survived until the OG timeline starts, there is no reason for her to die before FFVII starts. Logically, it makes no sense to have her die before that.
However, and I've stated this for more than a year already, the fact is that Zack being alive threatens the FFVII plot. I keep telling this, and people keep disagreeing for whatever reasons, but it's Zack dying that makes FFVII possible, it's because he dies that Cloud is able to walk by himself to Midgar and the chain of "good" events go on. Plus, FFVII has always been dark, in all honestly, yes it's bright and colourful, but the story has always been dark, the characters have been wounded in ways we can't conceive.
So Zack being alive is pretty much a butterfly effect; if he's alive, Cloud can't meet up with Tifa, he remains in a comatose state, with mako poisoning. Barret and the Avalanche gang die early. FFVII doesn't start. Aerith gets taken (at best, it seems worst have happened there) by Shinra, and Hojo doesn't have pretty plans for her anyway. Red XIII remains as a prisonner in Hojo's hands. This is plainly what happens if Zack remains alive. Obviously, Zack didn't meet up with other refugees from sector 7, so we can guess that *at least* the plate fall did not happen in his reality.