I believe Roxul is the brand of rock wool insulation I used. 10yrs ago, it was not well known in the PNW, not sold by Lowes, or Home Despot. I had to go thru a local DIY chain, and special order it. It was a little more expensive, but I thought it was well worth it. It came in different widths, and thicknesses then, and I wouldn't be surprised if it has many more custom applications, now. I used unfaced batts in floor, ceilings, and walls. But I used HD plastic sheeting as vapor barrier over insulation and studs, and beneath sheetrock, with a well ventilated attic. If your furred-out interior wall is just for holding up insulation, and drywall I wouldn't be too worried about structural issues. I mean it's not carrying the roof span load.
An added benefit of insulating the walls that we haven't talked about is noise reduction.