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In this case, Hans-Diedrich von Tiesenhausen became a civilian in 1947 (having been discharged from the Wehrmacht).
No charges as a war criminal were made against him between 1942 and 1947 (1942-44 in the UK ; until 1947 in Canada) while he was a prisoner of war.
Your cited source states:
Furthermore, in September 1950, the German nationals were removed from the category of enemy aliens.
In 1951 Hans-Diedrich von Tiesenhausen, togeather with his mother, immigrated to Canada.
No where is it stated:
It is unlikely that the Canadian Immigration didn't know that he was a POW in the UK since 1942 and in Canada between 1944 and 1947.
See the research below.
Obrist on Margolian, 'Unauthorized Entry: The Truth about Nazi War Criminals in Canada, 1946-1956'
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Instead, he argues, the blame is to be put on the war criminals and collaborators who gained entry to Canada by forged identities or by giving false information about their wartime history.
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I fail to see how your 'research' is relevant, since Hans-Diedrich von Tiesenhausen immigrated to Canada under his own name (togeather with his mother) and no one, in any way, shape or form, have suggested that he gave false information about his wartime history (especially considering that he spent 3 years in Canada as a prisoner of war).