I think you're right about the level of planning and that he disposed of her body the night before.
In the past, I'd thought unlikely Megnut's suggestion that BM may have used the corpse of an elk (the antler rack which he displayed in his yard) as a coffin for SM's body, but I've changed my mind after learning of that tranquilizer dart cap in the dryer and those discarded hiking boots (laces thrown away at a separate location so as to not have the boots and laces - which likely have his DNA - found in the same place).
I think it possible BM may have had a hole pre-dug somewhere in those mountains. He may also have baited, tranquilized, pre-killed, gutted (and removed the rack from) an elk, the corpse of which would later become SM's coffin. And he may have temporarily buried that elk corpse under a small tree that came in a tree container.
Then, he may have tranquilized SM at the home, placed her sedated body in that now-empty tree container in the back of his rig, drove both up to where he'd temporarily planted the tree and elk corpse, opened back up the hole and removed the tree and elk corpse, dragged SM out of the vehicle and shot her to death, stuffed SM's remains inside the elk's corpse, buried that in the pre-dug hole, and then planted the small tree back over the top. And then he tossed the tree container over a ridge.
Perhaps he didn't even shoot her before he placed her inside the corpse and buried her under that tree. Maybe he liked the idea of SM smothering to death inside that elk corpse under that tree.
The deer farmer-hunter-landscaper's solution to "how to get rid of your cheatin' wife's body so it will never be found"
MOO
