Hi saml,
Welcome to the Mattress Forum! 
You’re very welcome.
I’m glad you took the time to phone Cozy Pure directly and receive direct and personalized advice. As you may be aware, they are a member of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. They are extremely knowledgeable about latex and different configurations, and you should be happy to know that the advice you received is neither “haphazard” nor does it “buck the general consensus of the site”.
Prone sleepers tend to require a bit of a “firmer” sleeping surface (see this article), and in the 10" mattress the choice that was recommended for you would be a common option. The nice option you have with the Cozy Pure is that they tailor the mattress to be two-sided, with a different comfort on each side. To achieve a different comfort, you have the option in this case to either flip the mattress over and sleep upon the firmer core, or you could keep the mattress in its current orientation and swap out the top layer. The direction you choose would be based upon the plushness/firmness you’d want to achieve and move toward based upon your current set up.
The uppermost layers of the mattress are generally labelled as the “comfort layers”, and those can be either hard or soft. In most cases, the layers move from softer to firmer as you move from top to bottom, with a thinner layer on top, in what I describe as a “progressive design”.
If you would flip the mattress over for a bit of a firmer surface comfort, you’d be creating a design that is what I term a “differential construction” with a “dominant layer” on top.
Dominant layering can have a very different “feel” to a more traditional progressive layering where the softest layers are on top. It provides a firmer sleeping surface and the surface foam compresses less than it would if it was a softer layer and tends to “bend” more into the softer layer below it so it can still provide good pressure relief and “give” under the pressure points (as long as the dominant layer isn’t too thick and prevents the softer layer below it from “coming through” or compressing enough).
It is a good solution for people who prefer a firmer, “crisper”, or less “mushy” surface feel or greater freedom of movement and don’t like the feeling of sinking in directly to the top foam layer as much but still need good pressure relief that comes from underneath.
The “feel” and performance of dominant layering can be “nuanced” or changed a lot depending on the thickness of the dominant layer and it’s firmness relative to the ILD of the layers below it and the thickness of the softer layers underneath. It can reduce the amount that the heavier parts of the body sink down into the softer layers of the mattress (especially if the softer layers underneath aren’t too thick) but still provide good pressure relief under the pressure points (again depending on the thickness of the firmer layer). So it can be a way to increase some surface plushness without the product allowing you to sink in too deeply initially.
In some ways it’s similar to an innerspring that has a variable spring rate that is softer with initial compression and firmer once the soft section of the spring is compressed and that has one or more layers of foam above the spring that are firmer than the softer part of the spring. It would be fair to say that it’s a different kind of softness with firmer overtones.
Some examples of other posts that talk about dominant (or dominating) layers in one form or another are here and here and here.
Flipping over this mattress would be akin to having a three-part component-style mattress of medium/firm/firm, and then rearranging the layers to be firm/firm/medium (top to bottom), creating the same style of “dominant layer” arrangement. Post #7 here (latex monozone unique properties) speaks a bit to the unique properties of latex and how a solid piece can in effect work as a “zoned” product with the variations in the compression modulus and more weight is applied.
I would consider the advice sound per the reasons I listed previously in this post. No worries there. And overall I wouldn’t get too caught up in “labelling” certain layers.
The advice is anything but “gimmicky” and is quite accurate.
You certainly could do that, but the 7" mattress would neither feel the same as the 10" model, nor would it have the options for adjustability in comfort by flipping or through exchanging of the upper comfort layer. All of the layers of a mattress work together to achieve overall comfort, with changes made to the uppermost layers having the most noticeable impact in comfort.
While you’re not necessarily in a higher BMI, at 200 lb you certainly might find it more comfortable to follow some of the higher BMI guidelines where such a population in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter. The 10" mattress would allow for more of that than the 7", but again much of that can come down to personal preferences.
While I certainly understand (and even advise at times) a certain amount of skepticism when shopping for a mattress and wading through so much of the marketing material that is out there, some of your comments seem to be more cynical in nature, so I’m not sure of the reasoning for that, unless perhaps you’ve unfortunately had some bad experiences in the past when shopping for a mattress (which would be many of us out there! 
). Regardless, I wouldn’t have a concern with the advice you’ve received as it applies in general. Of course, in the end, only your own personal testing will be able to determine if a comfort of a mattress is appropriate for your own individual needs, and a manufacturer can only provide the advice that they honestly think will best meet those needs based upon the information you provide them and their experience with their own products and clients they’ve previously assisted with similar needs/somatotypes.
I know that’s a quite a bit of information, but I hope it helps clarify some of things you were told. If you have other more specific questions, I’ll do my best to be of assistance.
Phoenix