Hi robbiev3,
You are fortunate that you wrote the exclusion on your receipt because as you know the iSeries warranty for any mattress with a quilted top has a 1.5" exclusion and it may never get to that point and loss of comfort or support is not a warranty issue. I’m also impressed that the retailer allowed this and is standing by it.
That leaves you though with the issue of what to do next … and this is aggravated by having an exchange selection that is very limited. Because of your greater weight, the durability of the softer materials used in the upper parts of a mattress are especially important and unfortunately Serta (and the other major manufacturers) don’t make it easy or in most cases even possible to know the quality/density of the materials in their mattresses.
Your best bet in these cases is to use the thinnest comfort layers that provide good pressure relief for your side sleeping. This way there is less sinking in and mechanical wear and tear on the foam and less foam to soften. Part of the problem is that even if a mattress doesn’t show impressions more than the exclusion … foam softening and the loss of comfort and support can make the mattress unsleepable even if it’s not a warranty issue. As you know … warranties have nothing to do with how long a mattress will be suitable for sleeping on.
In addition to this … even those mattresses they make that do use better quality materials (if you can even find this out) have poor value compared to smaller manufacturers that use the same or better materials that sell at better prices.
Because of the ongoing issues with foam softening you have had … I would tend to choose mattresses that used higher quality materials, thinner comfort and “pillowtop” layers (and I would tend to avoid pillowtops completely) and firmer materials as well (firmer layers won’t compress as much and will last longer).
You can see the layer breakdown of the prodigy in post #11 here. As you can see there is a lot of thicker softer foam in this mattress as well and the odds are good that premature foam softening could also be an issue with this as well. Mattresses with thicker softer layers and higher weights don’t go together well. It’s also important to remember that all memory foam will go through an initial softening period and if you are already “on the edge” of your support needs when you buy the mattress it will likely cross over the line into having less support than you need very early in the life of the mattress (as is happening now).
If I was in your shoes … and assuming that you can only exchange with another Serta (and I would ask this because the exchange is being made based on the store’s guarantee rather than the manufacturer’s warranty) … I would be very tempted to exchange for the iComfort model with the thinnest possible comfort layer (which is the Insight model which only has 2.75" of the gel memory foam over the support core) and then add a topper to adjust for pressure relief and comfort. This would “eliminate” some of the more questionable materials of the higher models and give you much better value as well. If you added for example a 2" or 3" latex topper to this you would have a sleeping system that used much higher quality materials at a lower price than if the mattress already contained materials that had the equivalent quality.
If you are committed to using memory foam as your top layer … then I would add a topper that used at least 5 lb memory foam in the thinnest layer that you believed you could “get away with” for your side sleeping but bear in mind that even high quality memory foam will not last as long as latex and will soften more. Another option that may be well worth considering is the new slow recovery Talalay GL latex that Latex International has recently released that is available from one of our members here. This has the slow recovery feel of memory foam combined with many of the advantages of latex and your total sleeping system would be higher quality and a lower price than buying an upper end model of the iComfort. Of course the risk with this is that you wouldn’t be able to test it first to see exactly how it felt but the combination would certainly be better value and still give you the “slow recovery” feel you seem to like. It would also have the advantage that you could replace the topper when it was necessary without having to replace a whole mattress (toppers will wear out sooner than the mattress below them because they absorb more of the ongoing constant compressions).
The only “downside” to this is that they may want or “encourage” you to “move up” to the same or a higher priced replacement mattress so that you can pay the difference and they can recover some of their “profit”.
So the Insight plus a topper may be a way to make the most of a very limited selection of good quality and value replacements and make the best of your situation and also have the advantage of a more “modular” sleeping system as well 
Phoenix