Leftover mineral wool insulation

The facing shouldn't be there in the first place. The facing/vapor barrier should always be on the inside. ("Warm side")

Having the facing on the outside creates a moisture pocket - moisture that gets between the ceiling below and the facing above becomes trapped, and encourages rot and mold growth.

Remove the facing and then you can pile on more insulation to your hearts content. Lay it perpendicular to the existing layer.

Roxul batts do not compress a lot, well not nearly as much as traditional fiberglass. They are also pretty rigid. They can definitely go 6 feet or so in a wall cavity. I have used them to do what you are doing, when there aren't things in the way.

I am guessing you believe you don't have cross bracing or electric/plumbing or obviously this won't work. I would unroll a batt, cut to height size (possibly cut to width), push up slowly starting at top of batt. If it snags and is resisting I would cut it. Then push up with broom or something similar and just keep on going until you fill it. Even if the Roxul slides a little you can stuff the bottom to hold it. The stuff really compacts well and holds its place. Why I tend to use it in basement joists.

Also if you place a small screw in the top side of your batt (just push it in so it isn't hanging out) you can use a magnetic stud finder to see where you are at.