Hi insearchofsweetdreams,
I hope you are able to find out the information you need.
Spinal alignment and perceptions of firmness would vary with each person so for this you will either need to test a mattress in person to decide how well it matches your specific needs and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) or you could make an online purchase with a good return policy that would give you the chance to assess it based on your own sleeping experience without the risk that can sometimes be involved in buying a mattress that you can’t test in person beforehand and doesn’t work out as well as you hoped.
What is the criteria that you are using to decide whether a mattress is “safe enough” for you? Once you have decided on the criteria and/or the materials that you are comfortable with then it’s much easier to eliminate mattresses that don’t meet your criteria or that use materials that you would prefer not to have in your mattress. If you are using CertiPur as your “standard” then almost any mattress that uses North American polyfoam or memory foam or that is CertiPur certified would be “safe enough” in which case you would have a very wide range of choices. If you are using a different standard such as Oeko-Tex then you may need to consider materials other than memory foam or polyfoam (with the rare exceptions of mattresses such as the Gemma that use memory foam or polyfoam that are Oeko-Tex certified). There is more information in post #2 here about organic, natural, chemical free, safe, and green mattresses that may be helpful in deciding about the types of materials you are comfortable with having in your mattress and that can help you decide on the answer to “how safe is safe enough for me?”
This will depend more on the specifics of the memory foam and on its response time and temperature sensitivity and on the thickness of the memory foam and the type of material in the other layers below it (or even above it) than it will on the density of the memory foam alone. Any density of memory foam can have faster or slower response and more or less temperature sensitivity depending on the chemical formulation that was used to make it. There is more about the different properties of different types of memory foam in post #9 here and in post #8 here but there are so many different types of memory foam and different memory foam mattress designs that your own testing will be the best way to decide on all the things that you can “feel” on a mattress.
All slow response materials will have more of an “in the mattress” feel than fast response materials such as polyfoam or latex which have more of an “on the mattress feel” but again there is a wide range of different types of memory foam that have either faster or slower response rates and this will also depend on the thickness of the memory foam and the type of layers above and below it as well. This is another one of the criteria that you will need to decide on based on your actual testing.
Most people that prefer a more “on the mattress” feel tend to prefer faster response and less temperature sensitive materials than memory foam.
Phoenix