Is this plausible?
Would the US have the naval power to defeat Spain at any time in the period 1800-1860?
Presuming a southern secession happens later anyway, might Cuba become one of the first secessionist territories reclaimed by Union occupation?
1. In short, yes, but not likely due to changes in the American political situation.
2. Situationally, yes. Assuming you can get a bloodier, more expensive, and longer-lasting Carlist War in Spain (Maybe the Liberal government takes on a more anti-Clerical stance, King Ferdinand flips back and forth in his final years on the Pragmatic Succession, thereby mudding the waters on Isabella's legitimacy, the Liberals take on a bigger debt and try to push through more centralizing reforms, further alienating the outlying provinces and colonial authorities), you could get a Spain crippled enough and with enough of a lose of legitimacy in its colonies to get the Cuban elite who both believe in the ability of the U.S to defend their position, lack belief in Spain's ability to hold the slaves down and retain their current privlaged position, ect. to back a fillibuster expedition.
3. Eh, I think they'll take a bit of time to reclaim, given the logistical pain of cross-oceanic operations and the fact that Cuban forces aren't going to get tapped to defend the Continental frontiers.