Honestly, as an experienced seamstress who has made PLENTY of formal dresses in 8-10 hours, I don't believe it is in any way unreasonable to expect that the bridesmaid dresses could be re-made in that short of a time frame, especially with seeing as they would have had access to however many highly skilled seamstresses were necessary to get the job done.
Again, this story doesn't give the full context of the story. We don't know just how poor the fit of the dresses, specifically Charlotte's, was. And, as someone with 25+ years of experience crafting these type of dresses, I can assure you, sometimes it is far easier and faster to simply start over from scratch rather than make alterations to a dress.
I'm not going to fault Meghan for being dismissive of Catherine's concerns. I've been in that situation, as the seamstress, far too often, where someone who doesn't sew thinks that making alterations is the easier/simpler path when it isn't. I can believe that Meghan and Catherine had a misunderstanding/miscommunication because neither of them are experienced seamstresses and that they would have just been talking at each other, rather at cross-purposes, whereas had either Clare Wright Keller or Sarah Burton or some other experienced seamstress been part of the conversation, it could have gone much differently. As it is, I do appreciate that Catherine had the graciousness to apologize for the part she played in their disagreement. Weddings, or any other major event with formal dresses, can be quite stressful, and I'm inclined to think Catherine wanted ALL of the bridesmaids to look their best and not be a cause for criticism or complaint on Meghan's big day.