Hi Scallawalla,
I’m happy that your new combination is an improvement over your old mattress system 
, and certainly your new pillow could have had something to do with the improvement of the comfort in your neck and shoulder region, as a pillow can have a dramatic impact in that area.
Regarding “back pain”, there is some information about the many different symptoms/pains that people may experience on a mattress and some of the most common causes behind them in post #2 here that may be helpful. I don’t know how long you’ve been sleeping on your new mattress, and if the addition of the memory foam layer was done initially with your new mattress or done after the fact. There is always a period of adjustment with any new configuration, so you’d want to make sure that you gve any changes at least a few weeks so that your body could adjust and lose a bit of its “learned alignment” from your previous configurations (especially your old mattress).
Low back pains can come from comfort layers that are too soft, too substantial (too many layers), inadequate deep support, as well as from comfort layers that are too hard, so it would be up to your own experimentation to determine what might be the case for you personally. Also, your own level of fitness, flexibility, somatotype and personal sensitivity will come into play, and these are all things that I unfortunately can’t predict via an online forum, and all of this is part of the fun (and also possible frustration :unsure: ) of a DIY mattress.
If your first configuration was with the latex alone on the spring unit and then you added the memory foam after the fact, you could compare those two levels of comfort. If your back pain was reduced with the addition of the memory foam, you could consider another layer of latex, perhaps a 1" of medium, to place on top of the memory foam layer for a bit of extra comfort, or you could consider a 2" layer of the medium latex and replace the memory foam with that. If you noticed a worsening of your back pain with the addition of the memory foam, you may wish to consider a 1" or 2" piece of a bit firmer piece of latex, and you could try that either above or below your current 2" medium latex layer. On top would provide a bit more of a “crisp” surface comfort, and below would provide a bit more of a “traditional” progressive construction that would provide a bit more of a firmer feeling “bending in” between the upper two foam layers.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
Hopefully that information gives you a few ideas as to your next steps.
Phoenix