Is it permissible to watch a foreign series from the past to benefit from it, while rejecting the transgressions within it, and is it permissible to watch another (series) out of curiosity, also while rejecting the transgressions?

The usual TV series—whether foreign or Arab—are accompanied by reprehensible elements, and there should be no disagreement about their prohibition. One must fear Allah and keep away from them. Your benefiting from them does not justify watching them, for their sin is greater than their benefit. Intellectual gains can be compensated for by reading and conscious study, bearing in mind that truth is not known by men, but by Shar'i (Islamic legal) evidence.