Hmm, it would be interesting that in an alternate ACW how the operations in Cuba would develop.
As you say surely any attempt of rebelion before the ACW would be crushed efficiently (before the Ten Years Wars of 1868-1878 there were also independentist conspirations like of Las Pozas in 1852 so I think that the perspective to be annexed by de facto the South of the United States -the slaver part of the Union- could be of the little taste of an important part of the cuban population), I think but that once the ACW had begun we could see an uprising against the sugar plantation oligarchy of Cuba that surely would support the South in the ACW, with the Union Navy effectively dominating in great part the seas, and the fact that Cuba could be a base for the southern ships that try to broke the blockade, I think that an amphibious operation a la New Orleans could be very possible and if this counts with an uprising in Cuba, surely Cuba could fall quickly in the hands of the Union.
A possible and plaussible chain of events could be this:
1854 Vicalvarada in Spain, the progressist uprising of Leopoldo O'Donnell against the government of Sartorius, POD the skirmish in Vicalvaro in 28 june is a disaster for the progressists with O'Donnell wounded, with an O'Donnell with a mortal blessing (not according our standards but yes with the medical standards of the spanish medicine of XIX century) the Sartorius government believes that he could crush the rebelion easily, this overconfidence causes him to delay actions, Francisco Serrano and Domingo Dulce joins the rebellion.
O'Donnell dies but the two other generals proclaims the progressist uprising that like in OTL has great following in different points of Spain, at the difference of OTL, Sartorius has more followers caused by the doubts that after the O'Donnell death this uprising coul truly succeed: Civil War of 1854-56 (at the end a progressist victory).
With a civil war in Spain, news of conspirations in Cuba and border problems with Morocco in the zone of Ceuta and Melilla the Sartorius government considers to sold the island, decision confirmed by an uprising leaded by Carlos Manuel Cespedes and Pedro Figueredo against the cuba governor that follows the orders of Sartorius. The lema of this uprising in the zone of Bayamo and Camaguey is freedom for Cuba (or at least a great autonomy under a progressist spanish government).
To worse things for Sartorius another uprising happens in the east, this with pro-annexionists to US in command leaded by Porfirio Valiente.
Sartorius pressed by all these circumstances indicates his will to sold Cuba to the United States effectively solding this in december 1854 ("the Great Christmas gift" in words of James Buchanan) -He will not survive tho this decision and the extension of the progressist victories in Spain, in february 1855 he will be substituted by Narvaez in a desperate attempt of the moderates to reverse the successes of the progressists-.
1855-56 United States troops with support of Cuban annexionists crush the uprising of the Eastern Cuba leaded by Cespedes and Figueredo.
1859. The Kansas-Cuba Compromise: Buchanan, southern and northern politicians accords the enter of Cuba as a slave state and Kansas as free state (a decision but fighted by radical of both sides and by cuban liberals antislavery), violence erupt (although in a minor way than in OTL in Kansas).
1860. Failed expedition of John Brown to Cuba in support of antislavery faction. John Brown jailed, sentenced and executed by treason.
Abraham Lincoln elected in one of the most disputed and divided elections of the United States.
Secession of South Carolina.
1861. Secession of the Southern States (Secession of Western Virginia and Eastern Tennessee of the Southern States in support of the Union). Cuba joins the South.
1861 October Carlos Manuel Cespedes executed by southern authorities. Grito de Bayamo, pro-union, anti-slavery rebellion against southern authorities leaded by Salvador Cisneros.
1862 Blockaded by the Union Navy, the island lives in an authentic civil war between the East (pro-union) and the west (pro-confederation).
1863 United States Expedition in support of the pro-union uprising leaded by admiral Farragut and Jose Antonio Saco.
November 1863 Fall of La Habana.
1864 March. Last organized confederate resistance in Cuba ends.
1865 June. Kirby Smith surrenders at Shreveport, Van Dorn surrenders at Sant Louis. Jefferson Davis captured at Galveston when trying to flee to Europe. End of the Civil War.
1866. Because the cuban uprising and the anti-slavery feelings of the middle class Cuba gets another time the statehood (suspended during the Civil War), the first ex-confederate state in see returning its statehood.