It sounds like Riedel may have taken a lot of this from Marty Bell's book Broadway Stories, although it's likely that he has also spoken directly to Zaks about it. Anyway, it's covered in that book, with many quotes from Zaks. I thought there was also a quote from Mignini in there about how difficult it was on her, but I'm not finding it. It may well be in there and I just haven't found it, or perhaps it's in another book.
I did not see Mignini. I was not a great fan of that production, although I also did not hate it.