I think the fundamental question is in your post: Why would the killer even move the bodies (to any room) and expose himself to multiple risks? (e.g. Leaving more evidence about the house, exhausting or injuring himself en route, and extending time in the house).
I posed that question earlier, differently: Why not just leave the bodies where they had fallen? The more you move bodies, the more complex the scene becomes. The pool room as a choice is complex: a basement room, requires careful maneuvering, creates risk of losing a body in the water or getting wet. But the more puzzling question is why they were moved at all.
If I came to a house with the intent to kill a couple, I would do everything to minimize my risks. In and out quickly, with no moving of bodies.
I'm familiar with discussions about possible staging of the crime scene (misdirection) and posing of bodies. I just find it a very strange risk calculus to make.
