Maybe I should add about the changeling thing, lest I made it seem as it likely was so - Mary Beatrice gave birth in a room full of people - and I mean full! - basically it seems that anyone at court could wander in... Some people may have believed in the Catholic conspiracy, of course - but basically they needed a reason to disqualify the rightful heir. Anyway, yes it's interesting. Should you ever get to it, I hope you'll enjoy it.
Looking forward. And yes, it's true that any rumor that could be started to discredit a rightful heir would spread like wildfire in a time of uncertainty.
The superstition behind a changeling seems to have arisen after birth, when a baby or child was sickly. The idea (at least in some places) was that the parents believed their healthy child had been stolen by fairies, and replaced with a sickly child. I guess that helped parents believe that their child was healthy and living a good fun life somewhere with the fairies.
Would that fit the story of this baby? Just curious.