When DM was arrested, they didn't have the truck or a body yet. I think he was charged with unlawful confinement because, as Kavanagh
said, he entered the vehicle of his own free will but wasn't allowed to leave, since he hadn't returned home and hadn't been found. For that same reason, the charge remains and the 1st degree is added (rather than 2nd degree).
But I am curious why MS doesn't have the same unlawful confinement charge (unless we just don't know about it). If he was there for the confinement, he would get the automatic 1st degree charge as well, but shouldn't he also have both charges? Do they have enough on him to have the 1st degree stick, that they don't need the confinement charge? Was the planning and deliberate maybe on his part?
JMO