Just some more musings. Firstly, yes what happened to German civilians, particularly children from the eastern part of pre-war Germany is mind-boggling. In the earliest newspapers which were allowed in 1946 there are photos of groups of children from months old upwards with short descriptions like, "possibly 2 years old, found on corner of St., blond hair blue eyes. Does anyone recognise him/her". I once met a man who had no idea how old he was, where he was born even his nationality. He had been picked up in a ditch in the Ukraine, crying by the body of a woman, by a German soldier who was later mia, left with the man's family who fled from East Prussia, picked up again and left in an orphanage in Berlin. The sister of the soldier had written what she knew and sewed the letter into his coat. That was all he knew about his origins.
Back to your problem, and I do admire you for even trying! Almost unbelievable about the records, however you do seem to have traced the probable unit and I wondered if it might be possible to find where they "retreated" to when they left Czechoslovakia? They must have been stationed somewhere in Germany and this leads back to the possibility that his girl-friend was an expellee, or even followed him. My reason for stressing the physical German location refers to the letter saying he was "not coming home". That indicates he was thinking of settling down where he was - and would it be likely that he would want to stay in Czechoslavakia once they knew the Russians were taking over? Possible - but? Just ideas, but you do not seem to have much else. Best of luck anyway, if you think I could help further please make contact.