Regarding Castro’s brother as successor: Raúl is only a few years away from Fidel, at no time was even remotely nearly as popular, nor is he as skillful in working the crowd (internally or externally). He is Chief of Staff of the military, which does give him a good base in any power dispute, but the military is badly off these days. The likelier official succession scenario would be to give him figurehead status while a collective leadership of younger but well-established apparatchiks such as Alarcón and Robaina take over. At that point you then start worrying about ambitious mid-level Army officers deciding to take over for themselves, or about druglords throwing money at various Army or Party factions to take over on their behalf.
Whoever took charge, IF they got any sense at all would immediately seek a way to strike a deal with the internal opposition, for a transition to a legitimate government in order to give the investors peace of mind and get badly-needed foreign aid. If it’s the officialdom, they’d seek one that would NOT involve any sort of major “payback” phase; if it’s power-seeking Army or Pary factions, they’ll likely inflict the payback upon the losing factions.
Meanwhile the Miami crowd would be screaming and yelling even louder than ever for the USA to not accept any transition that does not involve having them be the ones who take over Cuba (and take their revenge), or even for the US to go in and shed some blood on their behalf taking Cuba back to hand over to them (fat chance). At the very least, we can be sure a lot of Cuban-Ams would be showing up in Havana demanding that whoever lives in the flats or runs the hotels that were built 25 years ago in what was grandpa’s farm 50 years ago, have to get out NOW! AND pay them rent for all those years.
The key element as to how long/short hard/easy the upheaval would be, is if the USA, upon the fall of the communist regime, would feel obligated to maintain a hard line on how things should go, or would aid and support any good transition plan to a democratic Cuba (into which most of our allies from Europe, the Pacific Rim and Latin America would likely jump in, carpetbags at the ready, whether or not we like it). I mean, there are folks in Miami with some political clout in the USA who will settle for nothing short of seeing every single non-defecting Cuban Communist Party official hanging from a lamp post, on fire, while being shot (AND get back grandpa’s farm, let’s not forget that).
In any case, if Cuba started to look like it would unravel completely into anarchy the US would end up intervening openly (maybe in a so-called “international” force, call it “Operation Prodigal Child”). Otherwise we’d play the factions, as usual.