Drake the RAF Erk, and the ARW...

re, case of Drake, the incompetent wife-beating black-mailing RAF Erk, killed by the ARW/ARP.

A. I did not get the particular significance of the shoe coming off Drake's foot.
B. How did any of the evidence such as said loose shoe, or the ARW being Drake's wife's brother, make him any more than another suspect of interest, rather than actually fingering him as the culprit?

So the shoe came off his foot, so what? It could have twisted off his foot when he was coshed with the stone by the surgeon...so it came off between there and the water, or during the second attack. They already knew that COD was drowning, that Drake suffered a second attack, perhaps by a second attacker, whilst felled from the first attack by the surgeon with a stone.
OK Drake bashes his wife, and the ARW is his BIL. Interesting person of interest... but proof?

I did not get it.