Since I am not wanting to make more hulls from this plug- I am proceeding towards a sacrificial plug :
Fiber / epoxy( so as not to melt plug with resin) the hull ( top and bottom)
After thickness is achieved, add acetone or similar to the inside and melt the blue foam out of the way. I may add another layer inside once I assess.
Trim up the openings - add combing around the perimeter of my engine compartment
Frame out as needed for strength and mounting of ballast, engine / boiler mounting plate , outdrive gearbox etc. epoxy in rudder tube and locate prop shafts and struts
Construct the lift-off superstructure
etc..
Sorry to fill your thread with is off-topic dribble.. I will not continue after this and will start my own thread FWIW- After looking at the torpedo tube pictures you posted - and posed the question---I have been scouring archives on early US , UK and French navy torpedo methods. The area under the "loading end" of your tubes sure look to be connected to a section of linkage as opposed to a tube to deliver pressure.
I am strongly leaning towards a cordite charge that is initiated by that "firing linkage" connected to the trigger hidden in the the protrusions you refer to.
So vote to cordite!!
Thanks again for sharing your thread on your wonderful and timely build and your friendly and helpful answers