Historical Carlos clearly wasn't any better at sensible plotting than the Schiller and/or Verdi version. :)

BTW, I don't know whether you're aware, but Carlos' half sister, Elisabeth's daughter, did later become Regent in the (Spanish) Netherlands (this was after the Protestant half had more or less managed to become independent). She was a patron of Rubens, and also his boss in his function as diplomat/spy at the outset of the 30 Years War. So your AU where Elisabeth manages to persuade Philip to let her join Margaret there instead of sending Alba has at least some sort of historical parallel. :)

Here's another fun anecdote for you, this one taking place several generations earlier. Margaret of Austria and her father Maximilian I. HRE had an extensive correspondence about everything from presents for the kids (i.e. future Charles V. and his siblings) to high politics. A bit background on the following priceless bit; Maximilian at this point has already been voted King and Emperor by the Princes Electors (succeeding his father Friedrich IIII HRE), BUT he hasn't been crowned yet. So he's usually referred to as "Emperor elect", not "Emperor". One reason why he hasn't been crowned yet is that the current Pope, Julius II, former Giuliano della Rovere (who shows up in any take on the Borgia story as Rodrigo Borgia's arch rival and enemy and in most takes on Michelangelo's life because he was the Pope to commission the Sixtina Chapel paintings from Michelangelo, then clashing with him about the execution of same all the time), isn't keen on the idea because hang on, Maximilian's grandson stands to inherit an awful lot of territory anyway, do we really want him to inherit the HRE as well? Also Julius fancies himself a "Warrior Pope" and indeed is busy conducting military campaigns in Italy.

Maximilian to Margaret, fuming: So, he's turned me down again. You know what I should do? Go to Italy, have him deposed and install an Antipope! In fact, I volunteer at becoming the Antipope! I'll show him who's a Warrior Pope. I'm totally ready to give up women for this. Hey, if I do this, maybe they'll make me a saint after my death and you'll have to pray to me, this cracks me up.

Margaret: ....

Maximilian: Wouldn't that be fun?

Margaret:....

Maximilian: I was kidding! Julius is still a pig for not crowning me, though. Signing off, votre bon père Maxi.

ETA: Ha! I've found a doctoral thesis about Margaret's correspondence which is online and actually quotes this letter in the original in a footnote instead of just paraphrasing like the biographies do. Check out Maximilian's excentric Renaissance French (he had learned it as a young man for his marriage with Mary of Burgundy, whereas for his daughter Margaret, it was literally her mother tongue):

„Et ne trouvons point pour nulle résun bon que nous nous devons franchement marier, maès avons
plus avant mys nostre délibération et volonté de jamès plus hanter faem nue. [...]affin que après sa mort pouruns estre assuré de avoer le papat et devenir prester et après estre sainct, et que il vous sera de nécessité que, après ma mort, vous serés contraint de me adorer dont je me trouveré bien gloryoes. Je envoye sur ce ung poste devers le roy d’Arogon pour ly prier quy nous voulle ayder pour à ce parvenir dont yl est aussy contant, moynant que je résingne l’empir à nostre commun fyls, Charles. De sela aussi je me suys contenté. [...]Je commance aussy practiker les cardinaulx, dont IIc ou IIIc mylle ducas me ferunt un grand service, aveque la parcialité qui est déjà entre eos.
[...]faet de la main de vostre bon père MAXIMILIANUS, futur pape. [...]P.S. Le papa a ancor les
vyevers dubls et ne peult longement fyvre.“