Hi ShoppingCanada,
Welcome to the Mattress Forum! 
Your questions amount to me essentially choosing what mattress might be best for you, which unfortunately isn’t possible to do with any accuracy via an online forum, as there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula to predict a style or specification for any individual that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here), because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress. But hopefully I can provide you some guidance about how to go about selecting a mattress and evaluate materials, and also address a few of your other questions.
The first thing I would have you do is read 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, 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 as well as the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for.
Outside of PPP (Posture and alignment, Pressure relief, and your own Personal preferences, 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.
I suggest avoiding any product where you can’t find out the complete details of the componentry, as that’s the only reliable way to determine if there are any weak links in the mattress. It is true that the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay, which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
Regarding purchasing a floor model, that would come down to your own personal value equation. Do mind a product that may have been on the floor for quite some time? Do you mind something that may have had quite a bit of traffic on it, especially in a large retail store like the Brick? Is the mattress warranty important to you (most floor models are offered without a warranty)? I would ignore the “regular” price listed and just look at the final price you’re paying, and then compare and consider that to other finalists you may be considering.
A higher BMI can present special challenges and generally requires firmer materials (in the support layers especially). This could be firmer latex or innersprings (the type of support component would be a personal preference and in the right design either could be suitable) or even a zoned construction. The same overall guidelines apply with higher weights though that PPP along with using high quality durable materials that will maintain their feel and performance for longer periods of time are the way to make the best choices. Heavier people in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal. I wouldn’t “rule out” any types of mattress and base your choices on your own personal testing. Post #3 here has more information and suggestions about heavier weights that is worth reading.
I read through that forum and had to stop after three pages :dry: , as there was just too much inaccurate and misleading information presented there as to make what is offered more harmful than helpful. Unfortunately, that site is a good example of why people’s opinions and "reviews’ of certain products are the least reliable way to choose a mattress and predictor of the applicability of a product for your own particular needs, regardless of how well-intentioned the people are leaving the feedback.
There is no specific material or component or type of mattress that is “good for fibromyalgia” in general because each person is unique, and a mattress that works well for one person with a specific condition such as fibromyalgia may be completely unsuitable for someone else with the same conditions to sleep on. In very general terms … softer and more pressure relieving materials that provide a more “relaxed” sleeping surface will tend to work better than firmer materials because for most people with fibromyalgia a softer more pressure relieving sleeping surface is a more important priority.
Materials such as wool or other types of natural or synthetic fibers or fiberbeds, shredded latex, very soft latex, or softer memory foam are all materials that have worked well for some people with fibromyalgia but no specific material will work well for all people with fibromyalgia. A forum search on fibromyalgia (you can just click the link) will also bring up more comments and feedback from others that are in similar circumstances that may be helpful (but it will also confirm that there isn’t a single “best” combination of materials that will work for everyone with fibromyalgia).
I would tend to avoid buying a firmer mattress and then adding a separate topper afterwards unless there is no other reasonable alternative or unless you have a chance to do some careful and objective testing on the specific combination in person using the testing guidelines in the mattress shopping tutorial. Deciding on a topper that would be a good match for both you and the mattress (the specifics of the mattress can make a significant difference in which topper would work best for you) can be almost as challenging as buying a new mattress in the first place. You can read some more of my thoughts about buying a mattress/topper combination that you can’t test in person in post #2 here.
Your largest selection will be online, and there is a list of online mattress sources for Canadians in post #21 here. If you want to provide your postal code I can see if I am aware of any better brick and mortar options near where you live.
When you can’t test a mattress in person and decide to shop online, then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc.) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs, options, and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
I know this is a rather long answer, but hopefully I’ve given you enough to get a go start with your search.
Phoenix