Hi Lackofzzz,
I’m glad to hear that a new pillow helped with the neck issues.
If your arm is falling asleep then the most likely cause (although not the only cause) would be a mattress that is too firm and needs some additional softness and pressure relief.
The “normal” break in and adjustment period would vary from person to person but would generally be in the range of 30 days or less but for some people it can take a little longer and some people don’t seem to notice it at all. If you have slept on your mattress for a few weeks and your “symptoms” seem to have stabilized and are consistent then it’s probably safe to assume that it won’t really break in much more or get much softer.
A more breathable material that allows for more airflow will generally be more temperature regulating (memory foam is usually the warmest of all the foam materials) but any topper you add on top of the mattress will have some effect on the mattress as well because it will reduce the amount of heat that reaches the memory foam which can make it feel firmer or at least take longer to soften with heat so if you add a softer foam topper (such as latex) that is more breathable for example then the surface feel would be softer but the deeper feel may be firmer although it would still probably feel softer overall. The only way to really know how any mattress/topper combination will feel for you “as a whole” would be based on your own sleeping experience.
A topper will also change how your mattress feels in other ways as well because you will “feel” more of the upper layers of a mattress and less of the deeper layers so if you add a more resilient topper such as latex then you would have a more resilient and “on the mattress” sleeping surface instead of the more slow response and less resilient and “in the mattress” feel of the memory foam.
Wool is also a great temperature regulator so you could add a thinner wool cotton/wool mattress protector (such as the St Dormier mentioned in the mattress protector post here) but it may also firm up the sleeping surface somewhat so it may help with temperature regulation but not as much with the additional softness that you may need.
It sounds like your sheets aren’t an issue (unless they are a very high thread count which may be less breathable) so adding a foam topper that was more breathable than memory foam (such as latex) would be the most likely solution that could help with both temperature and the additional softness you need outside of returning the mattress and starting over again to look for another mattress that would be a better “match” for you (which is probably what I would do rather than spending more on experiments that may not turn out as well as you hope for).
Phoenix