Hi fridadc,
Welcome to the Mattress Forum! 
I’m sorry to learn about your pains and struggles in finding a new mattress, and I wish you would have found use before purchasing your current mattress and receiving some of the “advice” that you have, but I’ll do my best to offer you some guidance. I hope it will help.
The first thing I’d like to do is help reset how to properly go about selecting a mattress.
Begin by reading the mattress shopping tutorial here. Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
In its simplest form … choosing the “best possible” mattress for any particular person really comes down to FIRST finding a few knowledgeable and transparent retailers and/or manufacturers (either locally or online) that sell the types of mattresses that you are most interested in that are in a budget range you are comfortable with and that you have confirmed will provide you with the all the information you need about the materials and components inside the mattresses they sell so you will be able to make informed choices and meaningful comparisons between mattresses and then …
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Careful testing (hopefully using the testing guidelines in the tutorial) to make sure that a mattress is a good match for you in terms of “comfort”, firmness, and PPP … and/or that you are comfortable with the options you have available to return, exchange, or “fine tune” the mattress and any costs involved if you can’t test a mattress in person or aren’t confident that your mattress is a suitable choice.
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Checking to make sure that there are no lower quality materials or weak links in a mattress you are considering relative to your weight/BMI range that could compromise the durability and useful life of the mattress.
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Comparing your finalists for “value” based on #1 and #2 and all the other parts of your personal value equation that are most important to you.
Regarding your current mattress, I don’t know which model you selected (Room and Board offers quite a few), so I wouldn’t be able to comment upon the specifications of this item. Also, I’m not sure if you’re able to return the mattress or if you’re going to have to keep the exchanged mattress that you are having delivered, so I can’t offer much advice in that area without more information, as it’s unclear to me.
As for the guidance from your PT and your physician, for a “firm” mattress, this is usually an indication that they desire you to have something that allows for very strong support and alignment, but it doesn’t necessarily mean that the item will have a “hard” surface comfort. There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
If you are very sensitive to odors, mattresses using fewer polyfoams or memory foam, and more natural materials, such springs and latex are common alternatives for people with Multiple Chemical Sensitivities (MCS). Regarding your latex sensitivity, you may wish to read through the latex allergy thread here and the many posts to which it links. A true latex allergy is quite rare. Post #49 here describes in more detail the three types of clinical reactions that occur to finished latex products.
Regarding a new mattress breaking in, there will be a break in and adjustment period for any new mattress or sleeping system as the mattress loses any of its “false firmness” and the cover stretches and loosens a little and your body gets used to a sleeping surface that is different from what it is used to (see post #3 here). This would typically be a few weeks but it can be shorter or longer depending on the specifics of the person and the mattress. Again, I can’t speak more about this because I don’t have the specifics on the mattress you selected. It is possible that you desire more surface comfort (being a side sleeper), but if the componentry within the mattress uses lower density foams or lower quality materials, using a new topper, while providing extra comfort, will be a bit of a band-aid, as over time the less durable foams will lose their comfort life faster than higher quality materials.
Overall, I’d need more information about your current mattress (or the one that you’re exchanging for) regarding the materials on the inside in order to give you more complete information, and then also I’d need to know if you are able to return any of these mattresses and “start over”, or if you’ll be left with the mattress from Room and Board. From there, I can provide more complete information.
Phoenix