Hi jkozlow3,
You wouldn’t need to do mattress surgery to add a couple more inches of foam to a mattress and a topper would probably be a better option for this. Mattress surgery is most useful if you want to remove some of the layers in your mattress because they have softened or broken down and need to be replaced and when there are no other suitable options available to you (see post #4 here). Based on your posts it seems that all you are looking to do is add some additional softness to your mattress which is what a topper is designed to do.
“Softness scales” are too subjective and there are too many variables involved for them to really have any useful meaning unless you have specific reference points of mattresses you (or the person who has put the scale together) have tested. Each manufacturer or person may have a very different idea of the rankings on a scale or where a mattress ranks on any arbitrary scale so I would use the topper guidelines as the basis for your choice.
At your weight and because you are looking for more softness I would probably lean towards the 19 ILD latex (because latex is my personal preference but I would keep in mind it may not be yours).
If cost is a more important issue because of the cost of latex toppers then a good quality polyfoam (probably 1.8 lbs or higher) in a similar ILD or slightly softer (polyfoam tends to be rated a little firmer than latex) would probably make a good choice as well and would be a lower cost way to experiment.
Phoenix