Yes, yes, only Havana was conquered. But European treaties dont care what is actually occupied. Spain can cede the entire island as part of the Treaty of Paris in exchange for keeping Florida, the island is British at that point whether the Cubans acknowledge it or not that island is painted red on maps. Definitetly a South Africa type situation will develop though I am sure.
Just because Spain says the island is British doesn't mean the Cubans would agree. I question if the locals would simply lay down their arms.
A huge number of the British/American invasion force died of disease within three months. I'm pretty sure that campaign was the bloodiest that the British suffered in the war.
An interesting potential divergence point might be the British efforts to enforce their will on Cuba. What if the British, in addition to or instead of taxing America to pay for the war, also impressed thousands of Americans to help conquer Cuba after the peace?
Imagine British regulars patrolling through the towns and cities of America impressing soldiers into a colonial army intended for Cuba where most will die of disease. That would likely be one hell of a bone of contention.