Hi issz81,
Yes … and even some of the “basic information” represents such a complete change in thinking about mattresses that it can come as somewhat of a shock. In most cases just reading the information as you would a good book rather than studying it as you would a textbook for a class you were taking is enough to “absorb” the basic ideas and will give you more meaningful knowledge than 90% of the salespeople who sell mattresses in the mainstream industry.
You are learning quickly 
How much information is available about any specific mattress will vary by manufacturer and mattress by mattress. In many cases you may find information about some of the layers (such as memory foam or latex) but not about others (such as polyfoam which is more difficult to find out and is often the cause of issues with lower density). Information sometimes can (and should) come from a retailer and sometimes from a search of the forum or with online searches if you are OK with the frustration involved with doing what a salesperson should be doing for you (and with some manufacturers coming up with dead ends anyway).
You already have the basic concepts which are either know what is in a mattress so you can make sure it has no weak links, or test a mattress that “fits” in terms of PPP and minimizes questionable materials (which are most likely to be lower quality), or choose a mattress with the least possible questionable materials (which is firmer with less padding) and then adding your own comfort layer as a topper that you know is good quality and durable. It’s a little more difficult to go the topper route because choosing a topper can be somewhat uncertain much like choosing a mattress (because the topper and mattress act together in terms of performance and feel) unless you can specifically test the combination together but a topper also has the advantage that you can change it if your needs and preferences change or if it wears out faster than the mattress below it without having to buy a whole new mattress. This type of modular sleeping system has many advantages even though finding the best combination can be a little difficult. Once you know the best combination then it’s easy to replace it for the same type down the road and a topper will also increase the durability of the mattress below it because it will absorb much of the mechanical forces that soften and break down materials over time. If you sleep on a mattress then you can use your sleeping experience to help guide you with the choice of topper along the lines of post #2 here and some of the posts it links to … but that’s probably getting ahead of the game for now.
This usually means that the softness/thickness of the comfort layers aren’t quite enough to “allow” the shoulders to sink in enough which can lead to shoulder pressure or alignment issues either in the upper body or sometimes in other areas as well. It can also lead to “twisting” away from pressure which can twist the spine in the area of the twist (such as forward leaning side sleeping or side sleeping where the lower body is more forward). It’s always the balance between the softness/thickness of the comfort layers and firmness in the support layers that leads to good pressure relief, alignment, and deeper healthy sleep. It can be complicated because the surface area of each part of the body relative to the weight of that part of the body and the changing surface area as you sink in deeper is very complex. For example the shoulders are lighter and often wider than the hips so they will sink in more easily into softer foam until the torso comes into firmer contact with the mattress and the surface area increases and further sinking in is “stopped”. It’s usually best to use your testing and concepts as a guideline rather than specific theory which can be very complex.
Hip pain can come from pressure points if they are resting on a surface that is too firm (like the floor) or it can come from sinking down too far which overextends the hip joint and stresses the tissues that are connected because they are no longer in a neutral position and it can even come from twisting of the hips. Any doctor will tell you that there are usually multiple causes for the same symptoms … especially when it comes to backs. If a mattress at one time was good and then you begin to develop symptoms then it’s usually either from foam softening or from the compression of fibers that have become firmer.
I can really only speak to quality/value and this depends on knowing the specifics of each layer which they don’t tend to list. In general no because most of the manufacturers they list don’t disclose the specifics of their layers. Suitability can really only be known by testing (or the knowledge and/or the knowledge and experience of a manufacturer or retailer) because theory is to complex and the information is too limited to use “theory” to make any specific recommendations for a mattress. In some cases Kingsdown is more open in Canada than they are in the States (where they would be on the “avoid” list) but you could test this by asking for the spec sheets for one or two kingsdown mattresses to see if they provide the information. In general there is nothing there that obviously stands out in terms of quality (mostly because information is missing). In some cases if a mattress has materials like latex there will still be differences in the type and blend of the latex and different quality levels but if there is a layer of latex in a mattress it would be better quality than other materials even if they don’t say what type it is … although latex in a mattress is probably outside of your budget range because it is a more costly material. It may be worth considering as a topper though depending on your budget and on whether you are able to test what it feels like somewhere and know you like it.
I couldn’t agree more … and they really don’t provide any meaningful information outside of marketing information and “stories” to their retailers either.
Phoenix