Imagine multiple sources of 115*F all pouring heat on your plant.. not good.
The surface temp of a Uniheat 72-hour bag is only 115* for about 4 hours, and then it starts dropping to about 100*. That's really not a very hot temp at all, particularly for a lowlander. As long as your plant isn't in direct contact with the surface of the heat packs, you should be fine.
Also, the temperature of separate packs isn't additive. Multiple packs won't cause the package temp to somehow raise above 100-115*. The only thing that adding multiple packs will do is cause the package temperature to reach 100-115* faster, and resist the external temperature longer. You will also definitely need that humidity bag you talked about, since that heat can really dry out a plant.
Mass: Since I do ship a lot of Neps around the country, I'm interested in how a heat pack did that to your plant. That looks a lot like frost-burn, but I've never seen what cooking a plant looks like. Was the plant sealed to keep from drying-out?