The pack strap theory is easy to test. If the pack isn't putting any pressure on her shoulders, it's not that. If the straps are too tight, tighten the hip belt and loosen the straps, adjusting the weight onto her hips. If that isn't possible, wrong size pack, or just the wrong pack.

My hands used to swell and do get tingly when it's very cold. Eventually the fingertips go numb. The swelling goes away if I use them while hiking - to hold something or use trekking poles. The cold is easy, wear glove liners/gloves appropriate to the temps, or hike with hands in pockets.

I doubt it's the blood flow theory....