I'm ready to switch to mineral wool bats for my double stud walls instead of HD cellulose.   Three reasons motivate the change, they are:  Difficulty finding a competent installer of dense pack cellulose in deep cavity walls.  Equal cavity r requires a deeper cavity with cellulose (12", r44.4 for cellulose vs 10.5", r45 for Roxul). Material cost, believe it or not!  To compare costs I used Lowes prices for Roxul bats and The National Fiber "expanded coverage" chart and a price of $11/30 pound bag, which is lower than that 'premium' brand can be bought for. & about as low as I could find for any brand  My calculated material costs  came out within 1%.  I'm framing 24" oc.  For the bats, I'd use r30, (normally used with 2x8s) in the outer 2x4 wall & push strips of r15 into the spaces behind the framing, both horizontal and vertical, followed by installing r 15 in the inner wall.  With the rock wool I'll have some gaps and cavities not a solidly packed monolithic mass, I doubt they will affect performance, does anyone know otherwise?  Also the rock wool clearly eliminates any need for fire blocking.  What are the down sides of choosing rock wool?   FWIW the Roxul costs $0.042/rsqft, Lowes EPS is $ 0.11/rsqft, Lowes 1/2" RMAX ISO is $0.138/rsqft. .

You are wise to be worried about the ability of a contractor to install cellulose in a deep cavity and have no settlement issues. This comes from a insulation contractor with more that thirty years experience. Unless you install the insulation by stacking the bags without remaving the plastic the 12" cellulose wall will settle!