It’s rather opaque at least to me who the ‘rightful owners’ are: willing buyer and willing seller might suggest indigenous Cubans, but I doubt that would do much about the blockade, and might makes right would presumably suggest that the party is right.
Moreover even if the ‘rightful owners’ of property were found, it’s far from obvious how that would solve geopolitical difficulties with the US; after all, all sorts of governments which violate most Americans’ notions of property rights are on very good terms with it.
As for the possibility of spontaneously choosing ‘democracy over the support of the Castro family’, at best this is an almost vacuous statement akin to ‘if nobody were to litter there would be no more additional litter’. Even quite praiseworthy and courageous peoples have some difficulty overthrowing their governments (e.g. in Sudan, Myanmar, Hong Kong…); the ‘patriots at the Boston tea party’ made similar choices, but merely choosing revolt does not guarantee success.
Finally, on the embargo: the medicines exception of course exists, but US sanctions can, if desired, affect trade with most of the world economy, since most companies have some connexion e.g. to the US financial system. To the extent that is not the case in Cuba, that is because the implicit suggestion of the original quote, viz., that were sanctions loosened certain economic benefits would accrue in Cuba, has itself either been implemented or not been contradicted by some contrary policy.