Hi NRI!
Welcome to our Mattress Forum! 
I’m glad you’ve found the resources on the website useful!
Sorry to hear about your Winkbed plus mattress causing you to have mid and upper back pain, and that its more uncomfortable than the mattress you were initially replacing!
Your higher BMI at 41.1 kg/m2 and your stomach and back sleeping positions present quite a few challenges. You were correct to look in the direction of more firmness as higher BMI sleepers generally require firmer materials in the support layers especially. It may be that the firmness of the 2" pillowtop comfort layers in combination with the 2"5 latex underneath would cause you to sink through to the firmer coil below and cause numbness in your arm. Depending on your sleep position and how close you sleep to the edge it is still not very likely that the edge would cause the arm falling asleep
I imagine you’ve already done so, but if not, it may be worth reading through our Mattress Durability Guidelines to get more information about the best materials and layer densities for a higher BMI. If your mattress is one-sided then I would look for 2.0 lb per cubic foot density or higher. It may be a blessing in disguise that you need to return this mattress. Especially as you seem to be shifting positions during the course of the night this mattress would raise some red flags at your BMI. They said on the chat session that the 2.5" Dunlop latex is 12 ILD and lower ILDs wear out much faster for high BMI individuals. Also, and the Plus pillowtop is 1.8 lbs which is very likely to break down much faster for a higher BMI.
Nest Bedding is one of our Trusted Members and like all the members here I think very highly of and consider them to be among the best in the industry. They use high quality, durable materials, and have exceptional customer service. If you contact them directly and discuss with them your situation, they’ll be happy to guide you in the best possible outcome.
I totally agree that with the large weight differential and sleeping positions between you and your wife (BMI 41.1 stomach/back vs BMI 20.3 kg/m2 back/side) to consider a split mattress that allows for customizing each side separately for an “Ahhh… yes!” effect.
A split left/right configuration would certainly have the best odds of success. The mattress can be built so that the total thickness of the mattress is the same for each side, which means that there would be no difference in elevation, and with a suitable tight fitted quilted cover, the “split” transition area between sides is usually unnoticeable for most people. You can read more here about side by side layering in post #2.
SleepEZ and Latex Mattress Factory are also Trusted Members on the site who specialize in designing and customizing beds for people with higher BMIs as well.
You may also want to try personalizing/customizing a side by side split layered latex mattress using your and your partner’s sleeping specs right on the SleepEZ website but I’d still make sure to call whoever it is you are considering to verify your choices. Like most of our Trusted Members – you may also find a discount code to TMU members which you can find on their member pages.
I hope this information helps you get back on track with a mattress that fits both you and your wife. I look forward to hearing an update.
Phoenix