Hi tripper2001,
Some of the information you are getting is a little misleading and not particularly accurate.
When you are testing mattresses locally then the only specs you need to know are the type and quality of the layers and components in a mattress (see this article) so you can make sure that there are no weak links in the mattress and make more meaningful comparisons to other mattresses. If a mattress uses 100% natural Dunlop from top to bottom then there would be no weak links in either mattress in terms of durability.
Testing a mattress for suitability (vs durability) is a completely different issue. The most effective way to decide on which mattress is the best match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) is with your own careful and objective testing using the testing guidelines in the tutorial post. There are so many technical details that can affect how well a mattress works for you that it’s much more effective to use your own testing to decide how everything works together “as a whole” (see mattress firmness/comfort levels in post #2 here) and I would avoid trying to decide on a mattress based on “comfort specs” such as the thickness or firmness ratings of individual layers or the mattress itself. Every difference between two different mattresses can make a difference in how it feels and performs and while the specifics of each layer will make a bigger difference than just the thickness of each mattress … in very general terms thinner mattresses will tend to feel firmer than thicker mattresses. There is more about the effects of thickness in post #14 here but again I would trust your body and your own personal testing to decide which mattress is the best “match” for you.
No. There is no “standard” definition of firmness between manufacturers so firmness ratings are generally only applicable when you are comparing mattresses made by the same manufacturer. Different people will also have very different perceptions on the same mattress depending on their body type, sleeping positions, and physiology so a mattress that is “too soft” for one person may be “too firm” for another. Firmness and softness is something I would evaluate on an individual level regardless of how firm or soft a mattress may feel for someone else or how a manufacturer “rates” their mattress.
The density of polyfoam is the most important factor in its quality and durability but it has very little to do with its firmness or softness. Any density of polyfoam or memory foam can be made in firmer or softer versions. Higher quality/density materials will also be more costly. There is more about all the many variables that can affect the durability and useful life of a mattress relative to each person in post #4 here and the other posts it links to.
I would also be aware that warranties only cover defects in the mattress (and only if the exclusions in a warranty don’t void the warranty) and have little to do with the useful life of a mattress because they don’t cover the gradual (or sometimes rapid) loss of comfort and/or support which isn’t considered to be a defect and in most cases will be the reason you need to replace a mattress (see post #174 here).
Since you are already aware of most of the better options in the area … I’m guessing that you’ve seen the Ottawa list in posts #2 and #3 here 
Phoenix