Stop the speculation, Gromit...........the police knew what they were up to. As to the vehicle colour, remember they had to start with the descriptions from the other kids. I suspect their interest was triggered by an attempted abduction in Aberystwyth the Friday before and they picked it up from statements and CCTV. As to the arrest, they probably wanted to get to the car and, justifiably, picking up DNA from it - as they need to have something to convince the Welsh CPS for murder. And then, would you want a potential murder running around on the loose, possibly capable of disappearing given his background, perhaps even moving in next door to you? Me thinks not.
As to her, my thoughts are that she is in the river or now down in the sea with the recent torrential rain. Allowing for what he has done, if he has done it, and if not professionally done, then it takes nine days for a body to surface - and they can be a hell of a way from the river estuary, depending on currents. We had that with an accident near Zennor 3 years ago, one of the victims surfacing near Padstow, some ten days later. I am speculating now, so I will stop.