Put smooth (almost slick) tires on your MTB and call it good. A 700C hybrid would work too.

I picked up a 1994 hybrid in 2008... still had the store price tag on it, so practically a time capsule. It had flat bars, twist shifters, etc. It's evolved considerably to something resembling a light tourer, including drops with bar end shifters. I ride the hoods most of the time, but when I get in strong winds the drops are nice. As you found with your wife's Schwinn, you can get some great bikes on CL. If you can find one in your size, you might be happy with it. In the meantime, consider smoother tires on the MTB.