Hi zeus1gdsm.
Welcome to our Mattress Forum. 
Thanks for sharing a little about your recent experience with a new mattress. Too bad you did not find The Mattress Underground before your purchase as you would have been better equipped to make an informed mattress selection. Sorry to hear your new 10" Primo cooling gel memory foam is causing back pains, and sorrier to hear that you’re unable to return or exchange it for a better-quality product. I did have a quick look at the online specs for this product and there are quite a few red flags.
The first suggestion I have is that you check to make sure that your support system “free-standing frame” is not moving, it is perfectly flat, and that there are no parts that are sagging or that are bending under the weight of the mattress and the people sleeping on it. It should provide similar support to having your mattress on the floor and you can test this by putting your mattress on the floor to see if it makes any difference. If it does then it’s possible that your support system could be part of the problem as well.
Here is a quick scan through potential issues with your current mattress, to help with the understanding of what to avoid or address with a future mattress purchase.
Primo International is a Canadian furniture company which seems to be sourcing their mattresses from China. Foams used in Chinese beds are mostly low/mid-quality materials … although they are now complying with safety CertiPur certified foams for Europe and North America requirements. They are most commonly sold under many brand names in big box stores and elsewhere. ( I see this mattress in HomeDepot, Wayfair, Mays, Amazon etc) I would also read post #6 here about mattresses or components imported from Asia or China and which may have been compressed for long periods of time in either shipping or storage before being purchased and a forum search on Zinus (you can just click this) will bring up more information and feedback about some of the brands and mattresses being sourced in China would make this somewhat of a risky purchase from the get-go IMO.
https://uploads.mattressunderground.com/outside/66/PrimoKalani-showssamespecsasPrimoSuperhttps://uploads.mattressunderground.com/outside/0https://uploads.mattressunderground.com/outside/1
Your higher BMI range and how these foams respond under higher pressure and temperature may also be part of the issue. Additionally, neither of the foams that are listed in this mattress has the density specifications (anywhere I looked) There are some things that you can know with certainty. One of these is that when it comes to polyfoam and memory foam that the polymer density of the foam (relative to other foams of the same type) is the most significant factor in its durability … regardless of the country of origin (see post #4 here about the factors that can affect the durability and useful life of a mattress).
Honestly, I think you are caught between a rock and a hard place. Adding a firmer topper won’t fix deep support, nor will it improve the longevity of this mattress.
“Support” primarily comes from a combination of the firmness of the deeper layers and the thickness of the comfort layers (how far away from the deeper firmer layer you are) while pressure relief comes primarily from the softness and thickness of the comfort layers. This is why the comfort layers need to be “just soft and thick enough” to provide good pressure relief in the most “pressure prone” sleeping position (usually the side for those who sleep in this position) but more than “just enough” can put you too far away from the support layers and not “stop” your pelvic girdle from sinking down far enough.
When you use a topper over a mattress two things will happen. The first is it will compress and in combination with the layers below it will take on the shape of your body profile. Softer toppers will compress more than firmer toppers. This “cradle” formed by the upper layers re-distributes weight away from the pressure points of the body. The second is it will “bend” into any softer foam below it. This “bending” will happen more if the topper is firmer than the layers below it (what I call a dominating layer) because the lower layers will compress more than the upper layers and the topper will “bend” into the compression (or any dips or soft spots) of the foam below it. This means that the surface of the mattress will be slightly less conforming and feel firmer (less pressure relieving) and you will have a more “on the mattress” feel than if you were sinking into a softer topper even though the next layer down is still compressing. Many people who are taller and slimmer will prefer a firmer feel like this because they are often be “sprawlers” and prefer a more on the mattress feel with more freedom of movement.
The downside to this arrangement is that the recessed gaps are filled in a little bit less than if the topper was softer and the lighter parts of the body (such as the shoulders) may not sink in as effectively for both upper body alignment and pressure relief.
I am afraid you are heading back to square 1… I would definitely explore contacting the vendor and negotiating for a refund … meanwhile, I’d recommend going through the readings I provided to help understand how to search for a more appropriate mattress.
When assessing any product, also be sure that you find out information listed here so you can compare the quality of the materials and components to the mattress durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
I know this is a recommendation “after the fact”, but you may be interested in reading about the different needs based on one’s sleeping positions as well as how overall body weight affects one’s needs and preferences. In short, people of average weight generally do well with average thickness and softness of the comfort layers and average support layers. And those with an above-average weight range do better with thicker and firmer comfort layers and firmer support layers.
No, but it is common to experience back pain on a mattress or foam layer that isn’t appropriate for your BMI or sleeping positions.
If the top 2" isn’t glued to the stiffer foam underneath and provided that the foam is above 2lb density for your weight range you could consider swapping it out altogether for something that may better suit your needs. But again, this basic construction may not be suitable for your BMI range. I’d say there are too many unknowns here to go down that road.
A higher BMI presents special challenges and generally requires firmer materials (in the support layers especially). This certainly could be firmer latex. Higher BMI people, in general, will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal. I wouldn’t “rule out” any types of mattress and base your choices on your own personal testing. Post #3 here has more information and suggestions about heavier weights that is worth reading.
It’s impossible for me to diagnose the source of any back pain, but back pain isn’t always an indicator that the mattress you’re sleeping on is too firm. Back pains in the morning can often be an indicator that the mattress deep support is too soft for your needs which would create spine misalignment issues.
Searching for a topper can be as complicated as picking a new mattress. There is more information about choosing a topper in post #2 here and the topper guidelines it links to, and there are some topper guidelines in post #8 here that you can also use.
I think you have quite a bit of reading, testing, and probably some hard decisions ahead.
Please keep us posted on your progress, and let me know if you have any new questions come up!
Phoenix