Hi Singsongster,
Unfortunately you are in a similar position to many people who have bought a mattress that isn’t suitable for their needs and preferences but are also “trapped” into an exchange that is limited to what the store has available.
There are several other threads that deal with similar circumstances including this thread (and two other threads it links to) along with this thread, and this thread and my suggestions for you would be very similar to the ones in these threads. They boil down to choosing an exchange mattress with the firmest and least possible amount of lower quality materials in the upper layers (such as lower density memory foam or polyfoam) and then adding a topper of your choice to add your own softer comfort layer. This component system has the advantage of a sleeping system that uses higher quality and more durable materials in the all important comfort layers which will be more durable and can also be exchanged or replaced down the road if necessary without having to replace the entire mattress.
While this is not an ideal situation to be in to say the least … at least it will help you make the best of a difficult set of circumstances.
The iComfort insight may be suitable in some cases as a base mattress for someone who was lighter (it uses a medium quality comfort layer of 4 lb particulate gel memory foam and a lower quality base layer of 1.5 lbs) but I would not recommend it in your case because this would not likely be a very durable mattress for someone who was 300 lbs. The Coral Bay would be even worse (as you can see from the layering described here).
Hope this helps.
Phoenix