I am sure Paul will add his final word shortly, but as far as I recall, he never had a proper surname. Perhaps they were not needed in the first age.
He did not take Magda's name because he ended up going with Veilleur. As you pointed out, I am sure since she was unmarried and her father's last name was Cuza, that is what she would have gone by. Unless she changed it for reasons related to WWII.
The last name you are recalling was created for the movie I believe. I remember hearing it when watching the movie and thinking it was a weird thing to give a character a last name where the was none.
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