So far, if you merge 2 slots, you eliminate FM differences... although you can use some tricks for fuel amount or weight...
So, merging slots means identical FM and cockpits, but, in theory, DM could vary if you include things like added fuel tanks or guns as "ordinance."
In fact what you mention "internal fuel tank non-droppable" is just what I need for P-39s since some variants otherwise identical had reduced internal tanks but could be added back in the field. I am sure the "non-droppable" characteristic can be done, which opens some possibilities.
It seems to me that making internal (i.e., invisible), non-droppable fuel tanks, bombs or guns would be an easy trick. Just create an appropriately sized rectangle or cylinder in 3ds Max and do what's needed to get it into the game. No textures or no shadow model needed (it's internal), and no LOD work needed (after all, it's already at the minimum possible number of polygons). In fact it sounds like a really good project for a beginning 3d modeler.
Change of AI within same slot is not possible yet, we would need to modify AI class to let the game know that ANY pilot in ANY plane must behave as Kamikaze if it has X loadout.
This sounds more like an AI problem than a slot problem. The kamikaze option should be a loadout option, with a full load of bombs and possibly internally mounted bombs.
For kamikaze behavior, it might be possible to rework the code for the Mistel, Fritz X or Azon bombs and add "kamikaze/missile" it as a new waypoint option for aircraft in the FMB, with further option of target type - ship, ground vehicle, stationary aircraft, flying aircraft (for "taran" or ramming attacks), or specific map location. But that's another project.