Obsidian made a good point in regard to no one looking over at Zack during his entrance: sometimes not everything in a scene is worked through in a realistic style, the priority can be emotion and the drama of the moment.
Part of me wondered why there are even people in the church, because it instinctively feels more dramatic to me if the church is empty. Then I realised that an empty church isn't actually dramatic at all in this case, because Zack would just think, 'well, maybe Aerith isn't around at the moment'. So how do you signal that something mysterious and negative may have happened, so that Zack has an apprioriately crushed and confused reaction?
I guess a group of upset people can do that. Now, if it's the plate fall, all you need to tackle is 'would Zack know about this before getting to the church or not?'. I'm not familiar enough with the geography of Midgar to answer that, though. If it's not plate fall, could it be that they think Aerith is still captured by Shinra? This alternative seems less likely, though. Of course, there is another theory floating around...Aerith is dead in this timeline. Possible, but I have a tricky time seeing where that leads, story-wise. Obviously Zack would be devastated, and.........then what?