What are you trying to figure out? I personally think your proposition is largely only academic. By the way first you say you are wondering about bulk and tankers but now about containers. According to MARAD there are 24 US flag Jones Act eligible container ships. Most I am guessing are used for CONUS to PR or Hawaii or Alaska trade, I dont know for sure but I do not think there is much container feeder traffic between CONUS ports (said "not too much" not "not any" for all nit pickers). Why? Trucks and trains. Its cheaper to put the container on a chassis or a rail car and drive it from anywhere in CONUS than it is to load it on a ship in GOM as you propose and bring it to the US east coast. By the way many factories have been moved to Mexico so even foreign flag carriers are competing with trucks and trains to ship a box from Mexico to US east coast. Dont know the size of that trade but I suspect they still cant beat over land shipments. (think someone is already running a rail car ferry between Mexico and US GOM though).
There are only 3 US flag Jones Act eligible dry bulk ships. I dont think this is because of the cost of Jones Act shipping. That is probably just the size of the market. Not a whole lot of grain being shipped GOM to US east coast. Probably some coal and the sulfur trade. Maybe someone else on the forum knows what they are doing?
There are 48 US Flag Jones Act eligible tankers. They seem to be meeting demand for coastwise shipments of petroleum products. The extra handling you propose for bulk liquid cargoes would seem on the surface to only add to the cost even if you incorporate the lower costs foreign flag tanker and the increased risks and schedule impacts. The US flag Jones Act fleet also has a pipeline industry to compete with as would any contorted foreign flag carrier scheme. East coast refineries already get their crude by pipeline, foreign flag tankers importing it and now rail cars as well.
Any case for developing a US short sea industry of any size would depend on big economic factors like doing something about the structure of the harbor maintenance fee ( Ref), wages to be sure but also altruistic notions like cutting down on air pollution especially in NE US by getting thousands (millions?) of trucks off the highways. Not to mention the cost of repairing our roads for these trucks to wreck again. Yeah, yeah I know trucks have their own fees to pay but I wonder how that stacks up on ton-mile basis and if that is really enough to maintain our roads and bridges? If trucks are cheaper and in the absence of any hard law to reduce truck traffic the cheaper mode will win out. I'm sure the pro-truck lobby has their own politicians in their pockets too so good luck with that.
There is room for a real discussion about how Jones Act eligible ships, MSP, VISA and VTA programs all currently are set up and add up to a maritime policy or the lack of one but I suspect shipping via Cuba (or anywhere else for that matter) will not have any big impact on the industry. Shipping to/from Cuba even less. Other than geographic proximity why would a shipper select a US flag carrier over a cheaper foreign flag one to ship something to Cuba?
These ship numbers above are for self propelled vessels 1000 GT and above. Reference: US Flag Fleet Good summary on last two pages.