I think it was well known that the family of Hesse had that disease. In John Rohl's book, it was that reason that Vicky oppose Wilhelm's wish to marry Ella. Her fears came true when Irene married Henry and the disease came to her family. Most certainly someone as knowledgeable as Miechen will have known the possibility of that. It was speculated that was the reason for her promotion of the match, especially she wasn't a fan of the British Royal Family or anything Britain for that matter...