As Gøran say, these are different approaches, dissolve the SS which creates a substantial amount of waste or use reverese AR and passivate the ss with nitric.
The positive with the first method is that you are left with the washcoat and the pgms in a concentrate and it should be complete and easy to process further, downside is the amount of waste acid.
The positive with the reverse AR is it creates little waste, only what is needed to cover the substrate.
On the negative side it demands a lot of nitric (It can be used for many converters though), it may not dissolve all the PGMs and if it contains Rhodium it may need a additional treatment in hot concentrated Sulfuric acid.
And in the end you will have a lot of liquid to denox.
Mechanical "scrubbing" may or may not remove the washcoat, but I doubt it can be done to any degree of completeness.
I can't quite follow your maths, something are off, are you talking about percentage?