Hi Vicky,
It sounds like one side of your mattress has been broken in more than the other one and it’s probably just a matter of giving the “newer side” a few weeks so that it can break in as well (see post #3 here). With a topper on top of it then it may also take a little longer to break in than it would without a topper.
Once you have slept on the firmer side for a few weeks so it can also break in if both sides aren’t the same then I would also give Tom at Royal Bedding a call and discuss it with him since it’s always possible that for some reason one side was made firmer than the other and he would probably be able to suggest a solution that would let you sleep on both sides of the mattress and take advantage of the durability advantages of a two sided mattress.
I would also stick with the topper that works best for you on your specific mattress because with a softer mattress a thicker topper may end up putting you out of neutral alignment and 2" would be more of an average choice that would be less risky in terms of support/alignment (and as you mentioned possibly temperature regulation issues as well)
Phoenix