Hi Nash,
Unfortunately you are in a rather difficult position but post #2 here and the other posts it links to has some suggestions and to a few other forum members that were in a similar situation about how to make the best of your circumstances.
In essence … the best possible solution for most people would be to find a mattress that has the least possible lower quality materials in the comfort layer (such as lower density polyfoam or memory foam) and then add a topper as a comfort layer which can be replaced as necessary without replacing the whole mattress.
I think if there is any good news in all of this it’s that the insight you have is a very “basic” mattress that is among the firmer of the iComfort models and because it has only 2.75" of memory foam it may be a good candidate for a topper which would add a bit of thickness and softness to the comfort layers and may solve the problem. If you were to do this I would probably go with a relatively inexpensive memory foam topper from a big box store to experiment with (so that if you make a mistake you can return it) and see if this solves the issue with the back pain and the excessive firmness … or at least gives you some good indications that it is going in the right direction. I would probably go with a memory foam topper in the range of 2" and I wouldn’t worry about buying the best quality (because the mattress isn’t the best quality anyway). This way you have a way to experiment with some toppers to make sure that a little extra thickness will solve the issue before you decide on a more permanent solution or a higher quality topper.
In looking at some of the other firm mattresses that are available at Mattress Firm … there isn’t anything in the range of your budget that I would really consider there because all of them will have too much lower quality foam in the upper layers. If you choose a mattress that is significantly less than your exchange credit, you won’t receive a refund for the difference and I don’t think any of their more expensive mattresses are really worth considering as the base for a topper either (because none of them have good value or disclose the materials inside them) with the possible exception of the Adele which would cost you more in addition to any additional cost for a topper and I don’t think would be worth the extra cost either.
So in the absence of any good information about the contents of their mattresses and considering that there are very few good options for a firm or ultra firm mattress in the approximate budget range you are looking at … the insight and a topper may be the best you can do to make the best of a bad situation.
FWIW … dust mites can live anywhere that there is enough humidity and some dead skin cells (or other food) to feed on. While memory foam mattresses can reduce the dust mite population … it won’t eliminate them completely. A good mattress protector will also help deprive them of the “food” they need to thrive.
Hope this helps
Phoenix