I've gone back on forth on pre-planning. On one hand, you have the girls out of town on a camping trip. You have some interesting financial/guardianship behavior from BM shortly after the disappearance. There's some behavior on Friday/Saturday that could be suspicious as well (MG's statements about his state of mind Sat am, raking out the beach, etc.). He obviously did hide the body well.
On the other hand, you have a really crappy alibi that was clearly thrown together at the last minute. You have a changing narrative (mountain lion to "got in the river" to abducted) and odd FB activity that doesn't seem to be well thought out at all. Plus, LE almost immediately discounted the bike ride (which was the centerpiece of the alibi), which means that had to have been sloppy, too.
Oh, and it happened on Mother's Day. Which, if planned, is a whole new level of evil knowing what that means for his girls.
All that to say, I lean toward splitting the baby on this one and calling it "hastily planned". I think he knew it might come to this, had been considering it for some time (and had likely given most of his thought to what to do with a body), but something happened that sped up a timeline. MOO, JMO.