New theory, totally unproven and probably unproveable, but what one comes up with when stuck inside all day, staring at a substantial snowfall:
Okay, let's say that one of the missing adults arranges for another of the missing adults to be disposed of by a third party, and chooses the mountain as a suitable place of reckoning. The payoff: $32,000 in the brown satchel. Thus, two people (and Maisie) would then be taken back to Eufaula alive and well and the surviving adult could establish an alibi over the next several days, before reporting the other adult to have "gone missing."
However. The third party sees an easier, safer road to the pay-off, one which does not involve two living witnesses. Boom! Shoots all three, dumps the bodies in Lake Eufaula or at other convenient locations along the way. takes the $$$ in the satchel, goes about his or her business. Maisie winds up stuck in the truck, and the other $32,000 remains stuck under the driver's seat, no one having known it was there.
No witnesses, total per kill a bit over $10,000, which is probably much more than the going rate.