Hi w9oh,
Most of your questions are answered in the tutorial so I would start there but I’ll make a few comments in my reply here as well.
I don’t know where you are reading this but the only reason to replace a mattress every 6 or 7 years would be if you were no longer sleeping well on it after that length of time. This is the type of “information” that I would take with a big grain of salt because there is no inherent reason to replace any mattress unless there is a specific reason that you need to and this is the type of information that would generally be provided by manufacturers that wanted to sell you a mattress every 6 or 7 years (or less) and whose mattresses are made with the type of “built in obsolescence” that would require it because of lower quality materials inside it (and who hope that you believe that this is good advice).
Some will last much longer than this and some won’t even come close to making it for 6 years.
This would be correct for latex (where firmness is related to density if you are comparing the same type and blend of latex) but not for polyfoam or memory foam. You can’t feel the quality/density of polyfoam or memory foam and any density can be made in a wide range of firmness levels so lower quality materials can feel exactly the same as higher quality materials in a showroom or when the mattress is new … they just won’t last as long. There is more about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here.
These are called microcoils and you can read more about them in this article and post #8 here.
You may be referring to buckling column gel and you can read more about it in this article and in post #2 here and post #2 here.
Yes … a mattress construction that uses high quality and durable materials and is suitable for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) will last most people much longer than 6 - 7 years.
Phoenix