A few compression straps and anything can be secured to a pack. Could you not secure the sleeping bag to the bottom of the pack?

As for being too warm and sweating, judging by the amount of clothes you were wearing, this does not surprise me. 15 F is equivalent to roughly 9 C. Near this temperature which we experienced yesterday while out hiking, both of us only had a lightweight long sleeved shirt, windbreaker, lightweight running tights and one pair of pants. We were warm but not overly. Granted once we stopped for any length of time, out came a sweater and toque.

My dump bag is my jacket, which has almost armpit to waist inside pockets that fits extra gear, mitts and a toque.

A small 18 to 24 inch piece of fleece folded in half makes an idea butt insulator and has other uses as well if and when needed. This small size also makes for any easy fit in the pack.