I am not trying to be argumentative so please don't take it that way. I just find this very puzzling. The odds do seem astronomical for the results to be as you described. Mind you, I am not the final word on all things YB. We have a couple of them and have hatched a couple clutches of them. However, looking at the pictures of the hatchlings, I do not see anything that looks undeniably YB in heads stamps, flames, or belly markings. If they are not YB, the odds that an Ivory could throw 7 snakes that aren't YB would seemingly be as astronomical as the former notion.
Quite the mystery and I look forward to the final act.