I was also at the matinee today, but I read the whole thread and the articles, so I'll try not to repeat any information or just speculate.
During the scene when she melts, you could see her silhouette go down as she went through the trapdoor. Then, someone came out and carried her off to the side and someone else grabbed her hat. JLT stood off to the side for a few minutes and you could hear people talking behind the curtain saying "Call 911!" but I didn't hear anything about blood, just muffled talking (I had a good seat). Then JLT left the stage and the lights came up and the curtain went down.
My dad was waiting outside the theater at the time and saw "someone in a pettycoat and green body paint being loaded into an ambulance." He made no mention of the dress, so it was probably taken off because they wanted to check her ribs. Shoshana was wearing the dress (or her dress) during the curtain call, but not when she first came up out of trapdoor, so who knows.
After the curtain call, Joey stopped the conductor and said that this was "against protocal", but he just wanted to let us know that Idina had hurt her ribs and that she was strong and was going to be okay.
As for Shoshana getting ready so fast, we were waiting for a good 45 minutes. There was definitely enough time to do a passable job, but at that point, I don't think many people were paying attention something as petty as that.
I felt bad, my sister and I ran into some people later that night because that said they were going to see Wicked. We told them about what happened, and at first they didn't believe us. Then they kind of freaked out because they had come this weekend specfically to see Idina's last shows.
Thought of this eerie parallel during the wait: During the filming of The Wizard of Oz, the trapdoor didn't work right for Margaret Hamilton and she was burned on her face and hands when the flames shot up.
I'm going to believe Joey for now and hope Idina will be better soon.
Like a firework unexploded
Wanting life but never
knowing how