Hi crank,
First of all I have to say I’m very impressed with the Toronto research you have done and I think the biggest issue you will face is deciding between so many good choices rather than trying to decide between 'best and worst". When you are down to these types of choices … then all the objective, subjective, and intangible differences that are the most important parts of your personal value equation is the only way to make final decisions but you are in a very enviable position that it would be difficult to make a “mistake” in terms of quality and value and that the only thing that you need to find out for certain is how the mattresses compare in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences) with some good objective testing.
I think your experience with the manufacturers you’ve talked with reflects my own more limited experience with them on the phone and on their sites in terms of their knowledge and pricing as well as some of the forum feedback. This thread started by Hobotrader also has more feedback about some of the manufacturers there which also reflects your own experiences for the most part. Forum searches on each one will likely bring up more feedback yet about some of them (use the forum search tab rather than the site search) but overall you really do have some very good quality choices and some great value available.
I think 8 - 10 times is a real exaggeration but Qualux is a high quality “family” of polyfoams made by Carpenter with latex like characteristics (see post #2 here) but there is also a wide range of quality/density available in the “family” so finding out the density that is in a mattress you are considering would be important to make meaningful comparisons.
The Toronto list is in post #1 here but I think you have already uncovered most of the best “value” and you already have so many great choices that at best I would call the ones you haven’t out of curiosity rather than necessity. Of course from a more selfish point of view it would always be great to have more feedback on the forum about the ones you haven’t talked with as well 
In the short term I think this would be fine with perhaps a blanket under it for protection. While I would be hesitant to do this in the longer term because of the lack of ventilation and the risk of higher humidity and the mold, mildew, and dust mite issues that can go along with this … I personally wouldn’t have any problems with it for a few weeks although I would probably keep an eye on things and lift the mattress from time to time to check for any problems or dampness.
There’s not much that you haven’t covered besides of course personal testing and making sure you know the density of any polyfoam base layers. I would also make sure that you include the type of cover and quilting in the mattress when you are making comparisons because this can also be a significant part of its cost (cotton or bamboo with wool quilting for example can be more costly than just a stretch knit cover without quilting).
Other than that you are definitely on the way to a high quality/value mattress it seems to me and I just wish everyone had the same amount of good choices that you do. I also think your experiences will also prove to be very valuable for others in Toronto as well.
Thanks for all the great feedback
Phoenix