Hey there UNT,
Welcome to the Mattress UnderGround, thank you for the question as we are happy you are here and have come to the right place.
I dont want to sound contradictory at all, but he is my take on your situation.
There is a distinct difference between something that actually provides active cooling, such as an air conditioner, or a topper with air or water being circulated and temperature controlled, and something that simply makes you feel cooler.
With things like phase change materials, the basic idea is that heat is being transferred from your body into the material, where the material absorbs that heat as it undergoes a phase change. That can make the surface feel cooler for a period of time. But it isn’t actually “cooling” in the same sense that an air conditioner is cooling a room. Eventually, that capacity is used up and the material reaches equilibrium.
There are also materials that simply come into equilibrium with the temperature of the room. If the room is 68–75 degrees and your skin is somewhere around 88–94 degrees, putting your body against a material at room temperature is obviously going to feel cool initially because of the temperature differential. You can experience the same thing on a hot summer day at the beach. If the water is 80 degrees and your body is considerably warmer, that water can feel quite cool even though 80 degrees would normally be considered warm.
So I think the compressor/evaporator argument is a little too simplistic. Nobody is really or should be claiming that a PCM mattress is an air conditioner or that it is going to continuously produce a colder environment. Each have their place and they are accomplishing different things.
An air conditioner removes heat from the room and transfers it outside sending colder air into the room. Passive mattress materials are generally transferring, absorbing, redistributing, or slowing the accumulation of heat at the body/mattress interface. The terminology gets a little loosey-goosey because “cooling” is a much better marketing buzz word than “temporarily reducing the rate at which you feel warm.” 
And there is another piece to this that gets overlooked quite often: moisture.
Keeping someone comfortable isn’t simply a matter of making the mattress surface colder. Humidity, evaporation, airflow, and moisture management all play a role. Once moisture becomes trapped against the body or within the immediate sleep environment, perceived temperature changes considerably. This is one reason materials such as wool and horsehair can sometimes feel surprisingly cool even though neither one is producing “cold.” They are very good at managing moisture and allowing the sleep environment to breathe. Also, a topper like the Perfectly Snug Smart Topper works so well, not because it is producing cold air, but it is circulating room temperature air, which is many degrees cooler than your skin temperature, while the circulating air is reducing the moisture around you under the covers. The same principle as performance wicking clothing. The fabric wicks the moisture from your body so the perspiration droplets sit or rest on the fabric waiting for the air to evaporate it.
The mattress showroom issue is also worth putting into perspective. A cooler showroom certainly changes how a mattress feels. Viscoelastic materials in particular are temperature sensitive, so a mattress can feel considerably different at 68 degrees than it will in a warmer bedroom. It also means that someone lying on a mattress in a cool showroom isn’t necessarily experiencing the same thing they will experience after several hours of sleeping on it at home. That is simply one more reason not to put too much weight on a five-minute showroom test. An then there is the theory that mattress showrooms often do some of these things deliberately. Colder showrooms, enhanced versions of mattresses are some of the little tricks they use. Not all, but there are some out there.
As for zoning, I’m not in the “zoning is good” or “zoning is bad” camp. Zoning is simply another design variable. Much like a high loft or dual loft pillow, zoning can enhance the sleeper experience if certain areas need to be “boosted” and for others, zoning can complicate narrowing solutions when a mattress is not comfortable.
Whether zoning works well depends on the person’s height, weight, body shape, weight distribution, sleeping position, and PPPs - pressure relief, posture and alignment, and personal preferences. A zoning scheme that works beautifully for one side sleeper can be completely wrong for another. It’s not terribly different from shoes. Some people do well with more arch support, some with less. Heel-to-toe drop, cushioning, foot shape, etc. all change the equation.
And regarding the three mattresses mentioned above, I agree that all three are legitimate choices. I wouldn’t necessarily make the decision based on which one has the most impressive-sounding cooling feature or whether it happens to be zoned. They all have a thoughtfully designed build. The @DLX offers split internal comfort layers with over 8 options for firmness and feel between HD, Latex and Memory Foam options. Helix’s Elite Seriers can be augmented similarly in their king sized versions.
For a very tall side sleeper, I would also look beyond the standard queen/king/California king question. Something like Engineered Sleep or Nolah may be worth considering because they can offer some of the larger custom or less-common mattress sizes, including options such as the Nolah Alaskan and the Engineered Sleep Epic Series in Wyoming, Texas, Florida, Alaskan and Family Bed sizes.
When someone is very tall, those additional inches can be considerably more useful than another marketing claim about “cooling.”
And I would put a good pillow surprisingly high on the list as well. For a tall side sleeper, the pillow has to fill the space created by the shoulder-to-head geometry while keeping the head and neck in a reasonably neutral position. You can have a very good mattress and still have the entire setup feel wrong because the pillow isn’t appropriate. In fact, the pillow accounts for a very high percentage of the entire sleep system comfort.
The same goes for the foundation. A good mattress needs an appropriate, robust support system underneath it. The mattress, foundation, pillow, bedding, room temperature and humidity are all part of the sleep system.
So yes, cooling materials can make a difference. PCM can make a difference. Breathability and moisture management can make a difference. Zoning can make a difference.
But none of those things exist in isolation, and none of them should be confused with a plain old fashioned an air conditioner. If remaining less hot on your mattress, the more natural you go, the less hot you will likely feel, but that typically commands a higher price tag. In the long term, that is not such a bad thing if you can afford the up front costs. The argument there is if your comfort preferences change, or you simply like to change it up a bit every 5 -10 years, then the mattresses you have narrowed down are right where you need to be.
Hope this adds some food for thought.
Maverick.