With no other facts to go on this hypothesis is equally difficult to prove or disprove.
Not at all, just look what OP wrote.
One of my great,great aunt's married a Joseph Yang in Trembatschau,Silesia,Poland. The marriage would have been around the 1890's.
Trębaczów; a small village east of Wrocław. I’m sure there were many Asians there in 1890’s. There’s probably none there even today. Only a fool would suggest that it’s of Asian origin. It may even have Jewish connection which was dismissed by BB. Many Jews were given and assumed Jung as their last name presumably because they were young at the time when surname became obligatory. That sounds much more plausible than some Asian theory.