From the information provided it looks like Taylor has a case. If the Mauthner family cannot show that Alfred Wolf had the painting through false auspices, and if Wolf had the painting before the Nazis came to power in 1933, and Taylor can show a legal provenance dating back to Wolf, then Taylor appears to have a legally obtained painting.
Wouldn't a statute of limitations apply? Also, I'm surprised that the Nazi's would not destroy Van Gogh given his mental problems and imperfect representations. In fact, he was almost an inspiration to the decadent artists they despised.