Winnabago
Banned
There are so many things wrong with this. First off, the Aztec monarchy was not hereditary like you think, the emperors who took over after Motecuzoma IOTL (and there's no reason for it to be any different here) were competent and respected, unlike their predeccesor
My bad. It was actually his brother (who died of smallpox), and then his nephew (and remember, the Aztecs had Montezuma killed, why would they trust his relatives, in a family-oriented society?).
Anyway, all that “competent and respected” bit? Irrelevant. If the Spanish are besieging Tenochtitlan, the Aztecs are going to all have a common goal and mostly stand together. If the king is off with the army trying to recover the empire, and everybody is dying of smallpox, don’t you think people would say, “We killed the king with a rock and now his nephew says HE’S king? Lolno.”
OTL, Cuauhtemoc was trying to keep the city from being taken, not increase his popularity. Once Tenochtitlan isn’t under siege, he’s got to be popular or else the mob has him killed too. And again, that’s what the Aztec military was generally told to do at the time, it’s what they knew how to do.
Cortes’s army “completely gone” is ASB. Many of them had armor and/or horses, and people simply weren’t as good at killing each other back then. Some would almost certainly escape, and probably end up in Mesoamerican courts.
When most of their army was away, and a guy with terrible political decision-making skills was in control.that doesn't really constitute holding it down well considering they only made it in due to being invited in by the emperor and got kicked out in a single night with the majority being killed.
And the Aztec Empire did not solely consist of Tenochtitlan, I have no idea where you got that from.
The Aztec Empire did not directly control almost all of its territory. OTL, most of said protectorated territory abandoned it, except for Tlatelolco. I imagine this would still be the case.
If Cuauhtemoc was “de facto ruler” he would be a very unpopular one. It’s not like his reign was going very well: smallpox killed the last king, Aztec people killed the one before that, it’s starting to seem like gods and men are against kings, some of his family was a bit too Spanish-friendly, provinces in revolt, etc.