I don't know why distilled water would improve this, actually.

One thing I have figured out: the video talks about using flour between the two bowls. I did that, but not nearly as much, thinking it was a thermal insulator, and I have pyrex. But: I think now it was actually an absorbant, and a big part of the problem as been a lack of water evaporation.

So last night I rolled back half of the foil, leaving enough to support the towels, and this morning, there has been progress.

I also found some rock candy recipes talking about a week(!) to crystallise.

So there is hope this batch may yet be recovered.