I have some experience in dealing with the CWGC and MN casualties. But more importantly I know people that have more experience dealing with them and a proven track record.
You are correct that the CWGC require proof positive by means of primary records. We need the 1918 crew agreement [part of ships official logbook]. The log book would have been lost with the ship and the shore copy used to determine a list of casualties. There appears to be no surviving crew agreement at the National Archives Kew but I do see one held at the MHA Canada.
I will be away till tomorrow but will pick up on this when I get home unless anyone else can help.
"If Blood was the price
We had to pay for our freedom
Then the Merchant Ship Sailors
Paid it in full”