I'm not very familiar with either concept -- legitimacy of exercise or legitimacy of origin. All I see is that the Carlists were clearly being inconsistent. The Bourbons claimed the Spanish throne through a woman (Infanta Maria Teresa), just as the Hanoverians claimed the British throne through women (one of my pet peeves is the assertion that "Ernest Augustus would be the king of Great Britain today, under the Salic law." People who state this forget that the Salic Law would not have permitted his male-line ancestor, Elector George of Hanover, to succeed in 1714. After all, he traced his descent from King James VI/I Stuart through his mother, the Electress Sophia, and she through her mother, Elizabeth, the Winter Queen of Bohemia).

You cannot rightfully exclude females from the succession later on, after initially claiming the throne through a female(s) -- unless one is talking about the semi-Salic law. But semi-Salicism was never the law in either France or Spain, in either a DE FACTO or DE JURE basis. It was the Salic law in the former and the Castilian law in the latter.

In fact, I believe it has been mostly the German dynasties which have officially had semi-Salicism in the their house laws (Czar Paul of Russia copied, into the Fundamental Laws, the succession law of the German house his own wife had been born into). The kingdom of Two Sicilies presumably operates under this, as well -- but I'm not sure if it's a DE JURE thing.

It is a succession law which actually favors females belonging to junior lines (e.g. Blanca over Isabel) within a house, since one has to exhaust the entire dynastic male line, before permitting a female to inherit the throne (thereafter, the succession reverts to the male line). After all, the successor is the female most closely related (whoever that is) to the last male (she is treated as a male, for the purpose of succession). Such a law is not intended to do justice to women; rather, it is simply a convenient measure instituted to prevent the total extinction of the dynasty.

I wouldn't mind such a thing in Spain, had it already been the official law of the land. But Spain was not Germany, where such a thing was the DE JURE thing in four of her five kingdoms. The Spanish tradition had always stated that a king's sons precede his daughters in the succession; but his daughters precede his brothers (male-preferred primogeniture).

As it was, the Habsburg kings of Spain mostly didn't have brothers, anyway, since in each generation only one son survived to adulthood (not surprising, given the poor medicine in the times). But the kings did produce sons, so the succession from Queen Juan la Loca to King Carlos II was lateral (so the precedence of a king's daughters over hypothetical sons never became an issue).

And it just so happened that the entire male line of the Spanish Habsburgs became extinct with the death of Carlos II: the king had no children, male or female, or brother. So once again, the precedence of hypothetical daughters over brothers never became an issue in this case, either.

Perhaps the Carlists could be forgiven somewhat for forgetting their history, since male-preferred primogeniture had never really been applied in the house of Habsburg (or, for that matter, in the previous Spanish dynasties: I don't believe King Fernando II of Aragon had any brother, either).

Still, they should not have been allowed to forget the critical fact that the Bourbons had been able to claim the throne of Spain through a woman. It had to be a great inconvenience for them later on, when King Carlos IV produced several surviving sons, the eldest of whom fathered only daughters. Yet, when the male-line of the Spanish Bourbons eventually expired, they once again remembered their ancestor Maria Teresa, by pressing the rights of Blanca. Go figure ...