I would suggest you setup a 301 permanent redirect to the active site to capture any extra traffic. Thanks for your question.

The plural so often makes a better sounding storefront, but the singular keyword often scores 10X or more exact match searches per month. A 301 redirect would only generate what little type-in traffic the singular domain got, right?

   In my own experiments I've been able to score top 10 in Google for exact match plural keyword search, but disappear off the search results for (much more competition) singular (using 301 redirect approach). Wondering if building out the singular keyword domain could make a difference. But also, don't want to risk duplicate content penalties.

I agree, a 301 redirect will be your best option.