Was Carlos' instability that pronounced at this point (1556)? I thought it only cropped up later (or rather, was exacerbated by his fall).
Also, why is everyone going as though a Maximilian-line inheritance of Spain is a given? 1556 means Carlos likely marries Élisabeth de Valois/Anna of Austria (or even his aunt, Juana of Spain, which was also considered). Him having kids by any/all is not impossible. In which case, Spain will stay firmly with him and his line.
After all, Spain isn't an elective monarchy like the Empire is. So Carlos WILL be successor to Felipe, no ifs, buts or coconuts. Despite having had an "insane" monarch, Spain hadn't/didn't regard this as being a deal-breaker (in fact, I think if most western European countries did, we'd have been spared Henry VI and Charles VI in England and France, for instance).
Now, obviously, if Cawwos is already nuttier than an almond tree in season, we'd see a regency of a similar sort to that that had been enacted for his grandmother. (Karl V would probably wed the boy to his aunt, Juana, and appoint her as regent for her husband. A marriage to Anna of Austria is unlikely before 1562 at the earliest, and a 13yo queen-consort is unlikely to be made regent for a crazy husband. And I can't see Élisabeth de Valois being Karl V's choice to fill said role TBH).
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