Hello again, Seaweed
After the Allendale sinking, Donald on 14 May 1918, signed onto the War African. I found its log in the PRO Kew. He suffered an injury at sea off the US Carolinas on 9 September 1918 and was discharged from the ship into the Long Island College Hospital in Brooklyn NYC, his personal gear and pay allottments being place in the care of the British Vice Consul. Discharged six weeks later, he arrived back in Glasgow four days after the Armistice.
Taken with your data, his first sinking must have been between the Thornhill and the Allendale.
Bryce Fraser