Mineral wool on top of fiberglass. Density and compression

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.

Remember, heat rises. In the summer the second floor will be hotter than the first for several reasons. I found that after reaching a reasonable amount of insulation in the attic which helps more in the winter season, removing the heat from the attic is the best remedy. I installed a power vent running off a thermostat. You need to make sure you have enough vents around the eve of the house and that they are not blocked for this to work effectively. The cooler outside air will replace the hot attic air and as hot air rises, this happens quickly. It worked for me when I lived in New York, but now that I am in Florida it is even more important to get the heat out of the attic. Most Florida homes have wind turbine vents and several of them to make this effect work. Newer homes here have high ceilings eliminating attics.