Fuel Filter Replace?

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The return is mainly to prevent vapor lock. You could look up the factory service interval, but after 50,000 miles it doesn't owe you a thing.

It appears that the return line flushes actually flushes the filter..

Um, why would you think that? Both the main outlet and the return line are on the same side of the internal paper filter. The inlet line is on the opposite side of the paper filter. As Rusty noted, the purpose of the return line is to recirculate hot underhood fuel back to the tank to cool and to help prevent vapor lock. GM used this fuel return on most cars that had the possibility of hot underhood temps, mainly A/C cars and cars with high performance engines. At 50K, I'd replace the filter. It's cheap insurance.