1) If two cartridges with completely dissimilar energy levels can create the same result on the same critter; if two cartridges with identical energy levels can have entirely different results on the same critter, then why are we even talking about it?
We talk about bullet diameter and bullet mass even though two bullets with different diameter or different mass can create the same result and two bullets of the same diameter or the same mass can have entirely different results on the same critter. Does that mean mass and bullet diameter are absolutely meaningless? Or do they provide some valuable insight while not giving us the entire answer?
The "potential" to do work is irrelevant. I had the potential to be a brain surgeon.
Potential is pretty important.
You had the potential to be a brain surgeon, but not everyone does. If you want someone to be a brain surgeon, it's important to have an idea of their potential before you expend a lot of time and effort and money trying to prepare them for a job they will never be able to do. Does everyone with the potential to be a brain surgeon become one? Would everyone who has the potential to be a brain surgeon actually be a good one?
The answer is no, and all the same questions and observations apply to the question at hand. Potential does tell you something, but it's only a small part of the puzzle. It is important to understand because you don't want to try to get blood from a turnip, so to speak. If a particular caliber doesn't have the potential (energy) to do what you want it to do, say expand and still penetrate 12-18" reliably, then maybe you want to look at a cartridge with more potential rather than expending time, effort and energy trying to make the cartridge do something it really can't do. Now, just because you pick a caliber with the potential to do the job, does that guarantee success? No, it doesn't. It just gives you a chance at it.
That "energy dump" accomplishes two things: the poking of holes, and the conversion of kinetic energy to heat energy--simple physics. The heat is not a player in this.
I don't like the term "energy dump" because it oversimplifies things. Kinetic energy is what is expended when a bullet makes noise when it hits the target, when it makes holes by damaging/displacing tissue, when it generates temporary cavity, when it breaks bones and when it deforms itself.
It's not like there's a load of energy, it all gets dumped into the target and the energy itself all goes to doing one thing. For example, the energy used up expanding the bullet doesn't really get dumped into the target at all, it "goes into" the bullet resulting in deformation of the metal and ultimately in heat generation. Once the bullet is expanded, it is considered more effective, so we see the loss of energy required to expand it as beneficial, but all the same, that's energy that won't be available to damage the target. An expanding bullet won't penetrate as deeply as a non-expanding bullet with the same mass and impact velocity, even after one takes into account that it has a larger frontal area. That's because some of the bullet's kinetic energy was used up deforming (expanding) itself and that's energy that is no longer available to generate penetration.
The best way to think of a bullet wound is as a combination of a penetrating wound (like you would get from stabbing someone with a sharpened rod) and a blunt trauma. The blunt trauma effect is from the temporary cavity.
The lower the energy, the less blunt trauma effect you get and at the very low end of the scale, it can act almost like a pure penetration wound. The blunt trauma effect in handguns is not enough to tear elastic tissues, but it is something that will likely get noticed while a pure penetrating wound can escape a person's attention in the middle of a fracas. More than once I've heard a stabbing victim say they thought they were being punched and only later realized that they were being stabbed. I've also seen a few instances where people were shot by lower powered cartridges and didn't notice until later. We can see then, that pure penetration can go unnoticed. A decent sized temporary cavity will feel something like being struck, and will cause bruising type injuries by the same mechanism that a striking object would. It stretches/deforms tissue and small blood vessels rupture when they reach their elastic limit. Nerves in the tissue are traumatized by the same mechanism, causing pain. Temporary cavity, in some circumstances can cause tissues that are not elastic to rupture but that's not something you can count on since that type of tissue is not uniformly distributed around the body.
What you can count on is that if the temporary cavity is sufficient, the attacker will almost certainly know they've been shot unless they are not in their right mind (crazy, drugged up, etc.). That doesn't do anything in terms of physical incapacitation, but the dirty little secret of handgun self-defense is that physical incapacitation is often not what ends up stopping attacks. In many cases, what stops attacks is the attacker's realization they've been shot motivating them to give up. One study suggested that over 80% of
successful defensive gun uses did not involve the attacker being physically incapacitated. Which means that it's very important for the attacker to get a "notification" that they have been shot and that's part of what temporary cavity does for the defender.
The penetration aspect of the bullet wound is what actually does the physical incapacitation, but the temporary cavity messes with the attacker's mind and helps with psychological stops.
This is why attackers who are crazy or heavy into substance abuse at the time of an attack can be ridiculously difficult to stop. It's not that the bullets somehow are less effective on them--that obviously makes no sense. It's that their brain doesn't get the "notification" that they have been shot and therefore they keep going until something they need to keep attacking actually quits working. They run out of blood, a big bone gets broken, the heart stops pumping, the brain gets scrambled, and they can no longer go on. We've all heard about the horror stories. But EVERY shooting can be like that if attackers don't realize when they have been shot. That is one reason why the "notification effect" is valuable even when there's no significant wounding generated by it.
Another reason temporary cavity can help out is sort of similar. Because it does get noticed, it is a distraction. I know I don't shoot as well (or think as well) when I'm being distracted, and I believe that's a pretty common failing amongst humans. Once a defender starts getting hits on an attacker, even if they aren't physically incapacitating hits, if the attacker is feeling those hits, it's safe to say that the distraction is going to impact their performance and give the defender an extra edge. Part of "winning" is not just getting the bad guy stopped, it's also not getting killed in the process, so anything that disrupts the attacker's ability to make good shots is a very good thing.
This is part of why focusing exclusively on wounding hasn't provided a good answer for the caliber question. When it comes to shooting human attackers, there's much more to whether the attacker stops and the defender survives than the pure "wounding" effect of the bullet.