I’ve seen more than one article that postulates the “Frankenberger” name is a cover for Rothschild and yes, the Rothschilds are of Jewish descent, even though they converted to Christianity for safety, business and political reasons in the 18th Century, IIRC.
Untrue. Most, though possibly not all, Rothschilds are still Jewish.
The (Karl) Marx family, however, did convert for the reasons you mention.