I want to understand both the mortality (were many people likely to survive that length of combat) and the likelihood (were many people actually enlisted for that long?)
Understand it may differ Western front v Eastern front.
Thanks :)
I will borrow some things from /u/flobota 's answer to a similar question about WWI.
While the Kriegsgräberfürsorge (akin to the War Graves Commission) existed in WWII and there was and still is (in different form) the Wehrmachtsauskunftsstelle (Wehrmach Infotmation Bureau, WASt) today known as the Deutsche Diensstelle that collect the information on those dead and those who served, the source problem is still imminent: The bomb that destroyed the archive in Potsdam in WWII also destroyed WWII personnel and unit records.
Thus establishing how many of the people enlisted in 1939 were still alive and fighting in 1945 is not really possible. As it is not really possible to research on a large scale who was enlisted when and released when and for what reason.
Additionally, the WASt during the war counted the deaths of about 3,2 million German soldiers, which has since been supplemented by an additional researched 500.000 deaths of German soldiers in WWII in the decades since by the Deutsche Dienststelle. However, the fate of 1 million Wehrmacht soldiers still remains unclear and they still are listed officially as missing in action. When Heeresgruppe Mitte collapsed in 1944, 100.000 men went missing. When there was fighting in Germany in between January and May 1945, 700.000 Wehrmacht soldiers were reported also as missing.
So both, mortality as well as length of service are really hard to establish in a statistical manner.
And as flobota wrote in their comment:
Forgive me for asking so bluntly (I am German), but why is the mathematical chance important for your understanding of World War I? It surely is a tough subject to wrap one's head around. But if I would say that 7% (made up number) of German infantry soldiers got through World War I unharmed. How would this change your perspective vs. 25%(made up number)? Is 25% a "better" experience for the soldiers involved?
Numbers surely are important to understanding history but they can only do so much. I personally like historians like Peter Hart who emphasize personal stories and primary sources from "smaller people" within these big historic events which make it easier to understand.