#1 You would not use decoupling clips to decouple an otherwise already decoupled wall. Just as you figured.
#2 The damped wall that is the heaviest will perform much better. Two sheets of standard 5/8" drywall is nearly 5 pounds per square foot. The 1/2" product you mentioned weighs 2.7 pounds per square foot. You'll notice this in the bass isolation right away.
Given that you'll have an anticipated 11% drywall waste factor, you can fill the dumpster with $7 a sheet scrap or $50 a sheet scrap.