No, I have not misunderstood your point.
Ena was a British Princess (not a Princess of GB&I) because she was granted the British style of HH, and later HRH, and her title of Princess recognised in the UK. She was not a British Princess because her mother was so. Because her mother was so, she was granted royal status, which is different.
That did not apply to the Spanish cases you mentioned. But you can compare her to children of Infante Carlos and Infante Alfonso of Orleans, who were granted the style of HRH. They were Spanish Princes, but not Infantes of Spain.
I think you misunderstood the main point of my post, which was to create a distinction between being a "Spanish royal" in a broad sense and an Infante/Infanta of Spain in a strict sense.
The reason for my bringing this up is that some time ago, I was told by a friend that the Spanish people regarded Queen Ena as having been born a BRITISH PRINCESS because her mother was one -- Spain being known to honor a person's maternal heritage -- all the while understanding that this was different from being a PRINCESS OF THE UNITED KINGDOM.
The latter, of course, was a legal (as opposed to adjectival) term: like all royal houses, Great Britain was entitled to set its own rules on titles and dynastic membership. As it was, Ena certainly did not meet the requirements to qualify as a princess of the UK, since she was not a daughter or granddaughter through son of a monarch -- like, say, her Edinburgh and Connaught cousins.
Still, if one is a Spaniard viewing the matter outside the box of legality (perhaps viewing the matter informally, influenced by Spanish cultural traditions), Ena could still be thought of -- at least in a loose sense -- a British royal. After all, she was born on the soil to a princess mother who had succession rights (and whose marriage was dynastically approved by the queen), she resided in the UK until marriage, she lived in the palace with her grandmother, she had a place in the royal court and took part in functions.
That was why I got to wondering if the same logic applied to the Sicilian and Bavarian princes who married the daughters of King Alfonso XII. After all, it's the Spanish system one is applying, when characterizing Ena as being INFOMRALLY a "British princess" who was not formally a princess of the UK.
No, in Spain, unlike other RH, for example the German ones, the title of Prince (Infante/a) only belongs to children of the Monarch and the children of the PoA. No member of the TS Bourbon has been born a Spanish Prince. The future Carlos III became King of the TS as an Infante of Spain and, even if he was the heir of his brother Fernando VI, he was never sworn in as Prince of Asturias. So, his children became Spanish titled royals when he became King of Spain.
Fernando of Bavaria was indeed the son and granson of Spanish Infantas and he was born in Madrid. But it did not make him a Spaniard or a Spanish Prince. That concept does not exist in Spain.