Hi LookingNow,
In addition to Jeff’s great suggestions there are also some suggestions in post #2 here and the posts it links to about mattress surgery that may be helpful.
If I remember correctly your mattress is made by Gardner and it may be inner tufted in which case you would also need to cut the tufting straps or cord. You won’t be able to replicate the inner tufting or tape edge the cover back together again without specialized equipment but a new zip cover that fits the layers and components will at least enclose the mattress materials and components and help keep them together although they will be looser than they were.
Doing mattress surgery on this type of two sided mattress would be more complex than doing surgery on a more traditional one sided mattress so I would proceed very carefully (and you will need to decide how to proceed based on the “exploratory” part of the surgery) but if I was in your shoes I would probably talk with Gardner about the possibility of rebuilding the mattress with softer latex and the cost involved rather than doing it yourself.
With a mattress like yours I would also consider using one or two toppers to achieve the additional softness and pressure relief you are looking for so you can “save” the mattress rather than cutting the mattress open and doing surgery.
Phoenix