While discussing the possibilities and impossibilities of exchanging Royal offspring and heirs in times of turmoil is amusing, it doesn't bring you much further in search for the truth. What you accept as possible is your point of view and so called proofs usually are pretty flimsy. Looking at the usual suspects, though, might give you some new ideas as to whether it should or could have been done or not.
Among the dwarf states of Europe, the Principality of Andorra is the largest. Until quite recently, it was an absolute monarchy. It was governed not by one, but two princes, and both are not citizens of Andorra. They don't even live in Andorra. To make the case even more curious, it is not a hereditary monarchy.