I can answer this easy since I am Haitian. Have Toussaint not captured by Leclerc or having Napoleon recognize Toussaint L'ouverture as the authority of the island. When Toussaint invaded Santo Domingo he entered the nation as a liberator not a conqueror. What that meant there was looting,no raping no destruction. Toussaint was loved by the Spanish Dominicans since he spoke Spanish and was a dedicated catholic. Having him alive would set a precedent on how to treat the Dominicans. Another reason why Santo Domingo broke away from Haiti was that Haiti has repeated civil wars,insurrections and was never stable. So a stable haiti could help. If Haiti was stable the dominicans would be wary of seceding. One way in preventing this have Toussaint alive and ruling Haiti. He would not chose Dessalines as his successor.
I also think a second war with France could unify the island. Let us say in 1825 during the negotiations with France Boyer says Haiti will not pay the 150 million francs. France sent 14 ship of the coast of Port Au Prince. It was clear to Boyer that were he not to concede to this "indemnity" that France would immediately re-open hostilities. I guess you can have Boyer say no or he dies and his successor breaks negotiations you could have a second war with France in 1825. If Haiti and Santo Domingo united defeat a French invasion it would be a factor to unite the nation. Plus Charles X was king of France. He was removed from power because of the July Revolution in 1830. Make the second war with Haiti a guerrilla quagmire then it is possible for the island defeat France. The mountainous terrain was a factor to why France lost the first place. So a second war with France from 1825-1830. A invasion would be enough to unite the two peoples to beat it back. Like when the Canadians beat back American invasions twice in the Revolutionary war and the war of 1812. So a second war against a united enemy could keep the two nations together.