Early on, I thought of photon torpedoes as rather like ball lightning. That they were drawn towards other ships without needing any guidance at all. By layering fields of different energies, you could give them different properties with no moving parts.
Here you have a new kind of magnetism
Here you have motion control with a laser.
So you have a wrapped form of energy, or perhaps anti-matter. This becomes entangled like a large romulan plasma torpedo in another ships field after a safe time has elapsed.
it is emitted from the lower dome and perhaps rides an invisible beam if need be.
An all energy TOW missile.
That sort of makes sense, to an extent. It explains the fireball effect at least, since the torpedo is a bolt of energy being externally guided. The "chiclet of death" sitting in the tube could, in that case, be a sort of "cable unit" that both generates and controls the photon torpedo through quantum entanglement or some such, adding or removing kinetic energy in one direction or another.
Though I'm grappling with the possibility that the casing itself is just a battery for what is essentially a weaponized shield generator. Turn up the shields high enough and they won't just repel things that touch them, they could smash/superheat those objects while also accelerating them away from it at fantastic speeds.
Basically, an explosion in a can. I'm just on the fence right now whether or not such a forcefield would actually require a physical object at the center of it or if that object could be left behind in the tube as an expendable (potentially rechargeable/regeneratable) cartridge. Considering that torpedo tubes more often are used like cannons than missile launchers, the latter seems more likely.