Hi bfg411.
Welcome to our Mattress Forum. 
I am sorry to hear of your pain on your 7 months old innerspring mattress. Too bad you did not find us before purchasing the topper in your effort to fix the mattress. Any softer layer on top of a unit that is not supportive enough is often unsuccessful as it does not address the issue at hand which is to keep your spine in neutral alignment during the course of the night.
Primary Comfort is connected with the IFD (Indentation Force Deflection) of the uppermost layers in the mattress that deal with primary comfort. The feel of “firm” and pressure points you are experiencing is most likely because of an incorrect firmness choice of the top layers
Pressure point issues and body soreness come from comfort layer(s) that is too firm or …. (unlikely in your case) too thin where you can feel the firmness of either the sleeping surface itself or the support layers “through” the comfort layers.
The primary support (deeper into the mattress) is most likely responsible for the “unsupportiveness right from the start” and the back pains that do “not go away in the morning”.
Part of the challenge you may have is if the issue is lack of primary support that it is too deep in the mattress for your BMI or …. too soft To better understand the difference I’d read post #4 for primary support, secondary support, and pressure relief and how they are related). Your attempt to “fix” support layers that are too thick/soft by adding the 2" soft Sleep on Latex layer on top can only be partially or temporarily successful because it would be more of a “band-aid” than a solution that “fixed” the core problem because the top layers can still “bend into” the support layers below them which lead to alignment issues.
Before any further decisions regarding this mattress, I’d also check
- If the foundation is adequate. I always suggest a “ground up” assessment to make sure that there is nothing under the mattress that may be contributing to the issue. Bed sizes above a twin should have good center support to the floor to prevent any flexing under the mattress and the people sleeping on it. In all cases … the mattress needs to rest on an evenly supportive base that will not sag or weaken over time under the weight of the mattress and the people on it. You can verify if the support system you are using is appropriate if you place your mattress/spring unit directly upon the floor to see if that makes any difference for you. if you feel any improvement then it is possible that the support system is the cause of your discomfort … or … there is a combination of faulty/flexing foundation and core layer being too soft for your prone sleeping.
- Did your mattress come with a trial period and what are the warranty policies that come with it?
I am not sure what bed size you have at 800 coils, 13.75 gauge How supportive the coils are depends on their diameter, number of turns but at your weight, it is unlikely that this may be the issue unless the unit is defective.
Your secondary point of focus is to get enough softness on the surface to allow your pressure points to sink in enough so that direct pressure on various parts of the body don’t cause soreness. Too much firmness or too much softness in either the comfort or support layers can both lead to alignment and pressure issues, discomfort, or pain in various areas of the body.
Quite right! A 14" mattress may be counterproductive for a low BMI range like yours if the mattress does not have the correct balance of comfort support for your specific needs. Usually, lower BMI individuals do quite well with 8-10" mattresses, and comfort layers that are too thick/soft can be riskier in terms of alignment. Side sleepers with lower BMIs do well with around 2" - 4" in your comfort layer. You have 5"-6" in your mattress compounded by an additional 2".
I’d keep in mind that a thicker mattresses have a greater range of compression and would need progressively firmer materials to maintain the same level of comfort/support relative to any given sleeper in order to maintain a good PPP(Posture and alignment, Pressure relief, and Personal preferences). You can see my thoughts about the effects of thicker layers or a thicker mattress in post #14 here.
I agree with your approach of identifying what is wrong with this mattress before deciding the next course of action. I’d start with the “ground up” assessment I suggested and the readings about the primary/secondary support and how support and comfort are related.
In preparation for your next steps I would, if you have not already done so, consider reading this article on sleeping positions and our Mattress Durability Guidelines as well as a gander at our Mattress Shopping Tutorial.
Many of our Trusted Members have latex hybrids that you may wish to consider.
I always recommend before making any mattress purchase that you have a detailed conversation with a knowledgeable and experienced online retailer or manufacturer and provide them with good information about your body type, sleeping style, general preferences and history, some general information about mattresses you have tested and done well with, and any other specific information or circumstances that could affect your choice of a mattress.
Phoenix