I think the problem came in with Bianca's second pregnancy, just before she died. It turned out to be a phantom pregnancy, and Ferdinando I exploited that and said Antonio was a cbangeling since his pregnancy had lukewise been phantom. So maybe give Bianca a second pregnancy tbat isnt phantom and we might have a better chance at Antonio I (after all, he had the support of Philip II of Spain, and if Marie still marries Henri IV, support from France thereafter).