This might be pushing it a bit, but at least theoretically speaking, it's possible for Spain to keep at least southern Mexico up to the point of the Spanish-American War. Of course, if Spain still experiences the Carlist Wars in this scenario, then Mexico could acquire its independence much sooner.
The US was able to decisively beat Spain in 1898--something that I don't see changing in this scenario if those two powers will still go to war during this time. However, I'm not quite as sure if the US would actually be able to decisively defeat Spain in a war in, say, the 1840s. Someone more knowledgeable about this will have to answer this question.
If it wins, it probably grabs as much Spanish territory in the New World and the Pacific as it can actually take.
When do you think was the first time that the US could actually decisively defeat Spain in a war?
Also, Yes, the Carlist Wars won't happen if Isabella II is born male--or possibly even if her younger sister is born male.
When was this?
Possibly not a Spain that's at the peak of its prime, but a Spain that's severely weakened by the Carlist Wars is quite a different story.