Despite all the caveats, DNA testing has made a huge difference in my research:
Thanks to (and through) 23andMe, I was contacted by someone who turned out to be a cousin -- different last name, living in Brazil, but "Half identical, 672 cM, 23 segments" -- who's made me aware of (and put me in touch with) an entire "second family" that my uncle (born in Poland, living in Chicago) "created" while travelling regularly to Rio de Janeiro! (Which explains a story that my mother always told me about her brother's wife selling his business out from under him while he was away on one of his "business" trips!)
I am happy to say that -- through me, and thanks to a couple of 23andMe DNA tests -- a new generation of half-sisters have finally been connected with each other!
Lesson: Never, ever, ever give up!
Researching: GELBFISZ/FISH/GOLDWYN (Warsaw, Los Angeles), LEBENSOLD/LIBENSCHULD/and variants (anywhere), SZAFIR/SHAFFER/ and variants (Warsaw, New York, elsewhere in USA), KORN (Poland, Philippines, San Francisco), BORENSTEIN (Poland, USA, Canada), WERNER (Poland, Glasgow) ...
Additionally, in/from Poland: BAUMEISTER, BRUMBERG, GUTLIEZER, HOPPENGARTEN, JARECKA, KORMAN, LEVY, NUSBAUM, OPOSDOWER, REJZMAN, ZEMSZ,