I'm so with you. The so-called murder mystery feels completely random and tacked on - do we really care why a character we barely knew died in an accident? While all the juice has been in the character study, as you said.

My theory: the whole "flash-forward" thing is in actual fact Noah's fiction. From the new novel The Ascent, sorta sequel to Descent.

Whitney was supposed to share with the other daughter. Meaning at that time all the bedrooms were taken, as the kids were spending a lot of time there.

Cole and Luisa had apparently just arrived to town and checked in to the motel that afternoon (since they had not seen the mothers yet). That means Cherry had probably cleaned the room that morning/previous day after the previous guests had checked out (and thought she had placed the soap there then).