I agree these situations can be very frustrating, but more direct questions and answers would have solved the whole thing. First, as someone else said, if these plans were firm they should've been discussed the night before--"Let's go to X tomorrow, we'll leave at noon." Then, those plans should have been stuck to--if, by chance, everyone but one person were ready to leave before noon, you could certainly pop in to their room and see if they're ready, too, but if they aren't, no one should be getting upset, because the plan was to leave at noon, not earlier.
So, my first instinct is to defend Alice. I think it's reasonable to assume "a couple of hours" from 10am is noon--maybe not exactly high noon, "a couple of hours" is a smidge vague, but I don't think anyone should call her "late" at 11:30.
There can be a lot of nuances to these things, though. Why didn't Alice ever confirm the plans--"We're still on track to leave at noon, right?" Is she sitting there watching everyone else turn up with their outside clothes, and it never occurred to her that she might want to get a move on herself? If you know you take 30min to get ready, and you're to be ready by noon, it seems a bit risky to not begin until 11:30, if you could have begun earlier--there's no time built in for anything to go wrong or to account for slight differences in clocks. In other words she was already risking being late according to the deadline she herself understood, which I think is a bit rude when you have plans with a group.
And then again, why didn't anyone say to Alice, around 11 maybe, "Oh, Debra and I were talking, and we thought we might leave a bit before noon, if everyone's ready. When do you think you'll be done?" Everyone else seemed to realize the timeline had moved up--either it was discussed but Alice was left out, which isn't fair, or it wasn't discussed but was more coincidental, in which case they have no call to mad at her.
Actually, to me this is one of those unsolvable problems that come up when trying to herd a group of people in the same direction, and why I try to avoid doing things like this with groups at all. The only thing I've found that works is trying to cover my own tail by explicitly asking people when we're leaving, where we're going, etc., even if everyone else thinks it ought to be self-evident. And then whatever I've been told, I try to make sure I'm ready well before then anyway.