R19 fiberglass is low-density, and would perform BETTER with the cellulose blown over it, since cellulose is far more air retardent and doesn't lose R-value to convection currents with a cold-side up configuration the way low density fiberglass does, and it's opaque to infra-red, unlike fiberglass. Even if it compresses the 5.575" batts to 5.5", the batt would then perform at a more stable R18 across a wide range of temperatures.
To meet all of your ice-reducting goals the rigid foam under the rafters, and empty rafter bays, soffit vents cleared you'd need ~R20 on the inteior, but R24 would be better. A layered up 3.5-4" of reclaimed iso from the usual sources works. If you leave at least 1.5" between the roof deck and insulation, and the soffit vents are truly clear, with 2x6 rafters you could put high density R15 "cathedral ceiling" batts between the rafters or cut'n'cobbled 3" rigid foam (any type) and only 2" iso (R12) or XPS (R10) on the interior side of the rafters for similar results. Whether it can tolerate that much loss of headroom is your call, but a 2x6 with interior gypsum and half-inch roof deck is an R6 thermal short, and in this climate you'd need a whole-roof R of at least R20 to have much of an effect.
Got any pictures of the walls with the plastic + blown/batt?
[edited to add] to hit R55-R60 with cellulose/fiberglass/cellulose sandwich, the entire depth of the sandwich needs to be initially in the ~18" range, and assume it will settle to ~16-17" over time.