Cancun 2010. Cancun has a high number of travel agents per capita. You’re never very far from one . As shown on this board, any of them can get you on a plane to Cuba.
USA, doesn’t matter. Cuba will let you right in, at least they did in 2005 when we went. Cuba will even do you the favor of not stamping your passport so you can lie about it if you wish. That said, you will have an exit stamp from Mexico then a reentry with unexplained time in between.