A Game is a game is a game.
SH3-games only realism is sitting infront of a monitor, hitting keys on a keyboard and move a mouse around.
Anything else has to do with the Gamers personality created with his/her braincells.
Good post. Good written and understandable.
But i have no good reason found to do what you like to attempt. To me it looks like the same as try to get a working Deckgun to XXI. As of today i have not seen that in a Mod.
An additional AA-gun is not needed to quickly sink a ship in-game; try modify the Shells used. My own install need around 3 HE-hits with the 88-gun to sink a tanker. It has been modified more like an Arcade-game - 140 ships (6 complete convoys and a TaskForce) in about 6 RT-houres yesterday with Deckgun only.

Also the Shells used for AA-guns in the game are only good for Cargos on Deck, or very small vessels. Not for greater ships. So you have to modify the used shells. Keep in mind, that some ememy ships use the same shells. For example the small speedboats use 20mm. You must be shure that the enemy can not use the same shells that you use, or your boat will be drownded very fast when you get under fire. Also you must realise that AI-Submarines may use the same shells as you use. When those german subs are added to convoys, they can qickly sink the whole convoy with those modified shells, when the convoy came into sensor-range. You may never get a chance to get near to the convoy-location to see the rest of the smokes of the sinkin ships will produce. Maybe you get a chance to see the sub(s) when you modify the speed of the out-camera to move as fast as possible to the location. So you must also be shure that AI-subs do not use the same shells es you use.
At least a additional AA-shell can be added to the "20mmVierling_High". I have AP (normal) and AA (added) running.
To my understanding of a Warship-mod another or a 4th Warschip - at the same time - is not needed, as you can only use one Warship at a time anyway. So re-create another Sub without a Deck/AA-gun makes no sense to me.
Good luck!