Hi Wool9349, and welcome to the Mattress Underground 
First off sorry to hear about your gastrointestinal issues, along with the glaucoma and back pains…sounds very frustrating to manage!
I would say not to worry about the specs for the Natura Soma XI, as your sleep profile has likely changed over the last 15 years. You have created a unique solution to accommodate your health conditions!
As I’m sure you know, any mattress you choose will be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions) - thanks for providing those! You have indeed done your research! - and your PPP (Posture & alignment, Pressure relief, and Personal preferences). As you want to continue with latex, as you know from comparing the Mattress Specifications You Need To Know to the Mattress Durability Guidelines, latex is one of the most durable materials you can have in a mattress.
It sounds like the combination of time, your ‘DIY’ foundation solution, and change in your needs is resulting in your current bed not being as supportive as it was. Since you want to reuse the frame, you may want to consider slats, or some sort of bunkie board with holes, as it sounds like you might not be getting adequate air flow under the mattress with your current set up. Also keep in mind that using foundations outside of those recommended by the mattress manufacturer can result in voided warranties/exchange policies.
Aside from that, the local retailers and manufacturers you listed all appear to have good latex mattresses - you might want to consider price and warranty terms to decide. Basically any good reputable manufacturer will have good durable latex products…it boils down to how they feel for you when you test. Remember, Dunlop latecomer rubber and Talalay latex rubber have quite different ‘feels’, with Talalay being considered a ‘bouncier’ latex (think of the difference between angel food cake and poundcake), so that might be another consideration, as you try some mattresses in your area. Since you have a lower BMI, any good latex mattress should provide support, you just need to ‘zero in’ on the right comfort level. It’s hard to make any recommendations on ILD as comfort levels are quite subjective: another sleeper, same age, size, and health conditions may find a particular ILD too firm while you find it too soft!
You might want to try out Talalay as well as Dunlop to determine what might be a better feel for your back. You would probably want a medium firm to firm, but exact ILDs would be a personal choice; at least firm enough to allow you to use the same ‘incline’ system you have to relieve the pressure from the glaucoma, but not firm enough to exacerbate your lower back pain….and too soft of a latex and you won’t have the same effect. Anything from 6" to 9" should work for you, as long as the comfort layers don’t allow ‘Sinking’ and bottoming on the firmer layers below.
One solution might be to try some different firmnesses of all latex mattresses at a retail showroom like Sleep Country Canada that have a few locations in the Toronto area - this would allow you to determine the type and firmness of latex you most prefer, which would allow for more choices in online manufacturers, like MFC and the like. I do agree the St. Dormier protector would be a good choice and would help keep moisture, dust etc off the latex. Any good mattress manufacturer or retailer should allow a reasonable ‘sleep trial’ period between 90 -100 days, as it takes time for a sleeper to ‘break in’ a new mattress and likewise for the mattress to ‘get used’ to your body profile. Make sure you review the warranty of any mattress you are thinking of purchasing to avoid unpleasant surprises. You don’t give a budget you are looking at for bedding, but you might want to review the offerings at Takasa who have a wide variety of sheets and bedding, and with the TMU discount you may not have to ‘settle’ for a lower thread count or less cool sheets.
As you have a complex set of needs, please feel free to drop in and share your experiences once you make a decision for the benefit of other forum members who may be in similar straits. I do wish you every success in finding a new ideal mattress!
~ Basilio