You dont nessecarily need the Spanish-American War as a POD, from as early as the 1850's Washington was looking into buying Cuba, however Spain's deminished sense of Great Power status meant they had no interest in giving up one of only two of their relatively decent colonies left (the other being the Philippines).
However maybe greater Cuban unrest, a weaker Spanish economy, a more successful First Republic in the 1870's could all lead to Madrid wishing to get a decent one-off profit from what was obviously a doomed situation.
As for Castro, what I've read about him suggests he was far from a commited Marxist-Leninist. The Revolution was led by his National Orthodox Party, more of a Social-Populist group than out and out Communist. However pragmatism led him into the Soviet Camp, after the US had something of a shit fit when he nationalised foriegn industry, plus Communism also offered him a strong platform to retain authoritarian control.
In a US Cuba, I can imagine Governor Castro to be far more like a Huey Long style radical, probably ruling the state as his personal fiefdom and along semi-legal lines.