The idea that the torpedo is caseless "in-universe" can IMHO be safely dropped at this point, except as speculation on Starfleet briefly flirting with a different technology during TOS and then returning to physical projectiles by the time of the TOS movies. But it is certainly possible that a photon torpedo would be a weapon type capable of carrying all sorts of warheads, the "photon" part being unrelated to the warhead - and that the warheads in TOS perhaps never were of the antimatter type.
Semantically, that would basically mean that a quantum torpedo is also a weapon type capable of carrying all sorts of warheads, and indeed such a torpedo does carry tricobolt warheads in "For the Uniform", even if only as "boosters".
But the ability of a "torpedo" to carry a physical payload from A to B is explicit in the quantum case, and implicit in the TNG photon case where the casing is known to be traveling in space after launch in a variety of applications, and something
physical and recoverable is known to be traveling in space after a standard photon torpedo launch. Even the TOS movie era has more or less explicit physicality-after-launch in that a novel guidance system is physically installed inside the casing in ST6:TUC.
In light of this, I'd loathe to separate the names "photon torpedo" and "quantum torpedo" from the warhead/payload type completely, especially when we also hear of things like "merculite rockets" or "pulse wave torpedoes" where the semantic connection is clearly intended (although we can of course dispute it). Of course, we also are supposed to believe in microtorpedoes, where the first part of the name refers to the properties of the delivery system, but that isn't exactly onscreen dialogue. In contrast, ENT dialogue on photon(ic) torpedo antimatter warheads is onscreen and explicit - but again we can argue it's but one (even if overwhelmingly the most common, and perhaps the only one in the 2150s) of the available warhead types.
In the end, "Dark Frontier" and the like considered, the glow still appears to be a propulsive thing. It's not the same exact type of propulsion as in the probes the E-D usually fires, or as in the starships and shuttles, but it might well be closely related anyway. Despite Trek being a visual thing first and foremost, looks aren't particularly relevant in this respect, because we already have several different established looks for a Starfleet piece of technology traveling at impulse or warp speeds. Does the exact same type of propulsive hardware move the casing at high warp in "The Emissary" without the trademark glow? Or is it different hardware? Or is it the same hardware, but on a coasting mode, having "burned out" long ago but still maintaining the projectile at warp?
The undisputable part of it all (feel free to dispute case by case!) seems to be this:
-Torpedo features at least two physically stored components: casing (all shows but TOS) and a separate warhead (DS9 "Tribunal") or several
-Torpedo is typically physically launched from a tube or a rack of some sort, even though we don't know why this would be necessary and whether the launcher plays a role in initial or subsequent propulsion
-Torpedo glows in flight, and cannot be made dark during powered flight or else stealthy firings would definitely take place (say, in ST6)
-Torpedo stops glowing after powered flight, becoming a difficult-to-locate physical object that can e.g. be studied for postflight establishing of flight parameters ("Genesis")
-Torpedo can detonate without being launched or made to glow
-Torpedo can maneuver in flight, and has an onboard tracking system at least as an option
-Torpedo can hit targets at both sublight and warp, although the speed at which it leaves the launcher does not appear to vary
-Torpedo tactical range is markedly shorter than the propulsive (or warp-coasting?) range of a torpedo-type casing, even if we don't know the exact specs of either
-Torpedo warhead can probably be removed, can explicitly be accompanied by add-on warheads, and yet is not explicitly known to be utilized without the casing part (although we can always speculate on any random demolition device being a torpedo warhead at heart)