Hi chip,
While a mattress can’t help a medical condition … it it keeps you in good alignment in all your sleeping positions then it will give your back a much better chance to rest and recover and your spine a chance to decompress and rehydrate which can help and support any other treatments you are pursuing.
Again this would depend on whether it is keeping your spine in good alignment in all your sleeping positions and allowing your back to completely relax without any tension in a neutral position. The testing guidelines in post #1 here can help you test how well it is working for you in terms of support and alignment. If your back issues are in the lower back then you would want to pay particular attention to making sure that your pelvis and hips aren’t sinking down too far which will tilt the pelvis and put your spine out of its neutral alignment.
This can happen for some people yes but it would probably be more common to "twist’ away from the pressure under the shoulders and sleep in a half stomach half side position. Each person can have a very different sleeping style and react to pressure or alignment issues in different ways. In general though your body will try to compensate in some way for pressure or alignment issues by tightening muscles or changing positions to relieve the parts that are under too much stress just like when you are injured other parts of the body can compensate and take over which can sometimes create new issues.
Not necessarily. There are no “rules” because each person’s physiology is different and back pain has many different causes. In general though either side sleeping with the knees drawn part way up (not straight like a soldier) and a pillow between the knees or back sleeping with a pillow under the knees (or with the legs elevated slightly on an adjustable bed) would relieve back tension and decompress the spine most effectively for most (but not all) people.
For neck pain the first place I would look is the suitability and support of your pillow. Neck pain could come from twisting the head and neck when you sleep. This would be especially true for stomach sleepers (who would generally do best with a very thin pillow or no pillow at all)
Phoenix