That's a gruesome thought, but not implausible, and could certainly account for sufficient forensic evidence for the CPS to be confident in the murder charge.
I have thought from the outset that the river system would have played a major role in the tragedy. Somebody posted earlier that they thought the Dyfi was too shallow, but in full flood it is often more than 3 metres deep at Machynlleth Bridge (which is the only obstacle before the open sea). It seemed to me to be clear from the fact that many of the searchers had to secured to ropes held by people on the banks that the river is treacherous when full and running fast. Also, the Dyfi is tidal up to a couple of miles below Machynlleth and, on the night in question, there was an outgoing tide after 10pm.
Even the Dulas, which flows from behind MB's rented cottage down to the Dyfi, is deep enough to have seen a man drowned trying to save his child back in 2010.
I think it likely that when MB was seen on the banks of the Dyfi, and later arrested close to the river, he was checking that no evidence was visible on the foreshore or stuck on the supports of the arches of Machynlleth Bridge.
Of course, one should only go on evidence, but I really felt that an innocent person would have expressed outrage or bewilderment at Aberystwith Magistrates Court when faced with the charges. To me, the tears seemed acknowledgement of the awful consequences of what was perhaps a moment of madness.