I should probably start with a little background.
Sloane, Ho and welcome to the Toilet Teaching board.
That sounds normal to me, and no you haven't screwed up with your DD by going back to diapers. With my oldest we started, and took many breaks in between going back to diapers, and then to underwear. For me after each break we took it it was easier because DS started right where we left off.
As for accidents, the first three to five days there can be numerous accidents, as this is a new process to them. After a week there should be a notable decrease in accidents, if there is not the child may not be ready for training.
For your DS continue with the training, by the sounds of it he very interested in wanting to. For your DD wait awhile and let her recover from her meltdown, she will let you know when she is ready. You could ask her in a week or so if she wants to try again.
You are off to a good start. I hope that helps. Please keep us updated on your twins progress.