Really nothing to do with dynamic award pricing. It's about married segment logic and the fact that you want to do F on one segment and Y on another. BWI-MSP-STL is cheaper than BWI-MSP because MSP is a hub and DL charges a premium on the non-stop prices (just as they do with revenue flights). Also, by breaking this up into an F on one segment and Y on another, you specifying two different award fares, one for BWI-MSP and another for MSP-STL. Just as if you did this with a revenue ticket, you end up with two fares (one for BWI-MSP in F and one the other for MSP-STL in Y). If it was all in F, or all in Y, they could quote you a single fare on the through routing. But because you splitting this into a F and Y combo, it will necessarily split this into two separate award fares instead of a single one. If you were to do this with a revenue ticket, you would almost certainly see the exact same sort of pricing (less expensive for a single F fare, than a combo of F and Y fares).

The mixed cabin awards are really meant for roundtrip bookings, not one-way routings like yours. In either case, it will require two separate award fares. It was basically enabled by DL publishing one-way award fare bookings. There is no special set of fares just for mixed cabin bookings. If you are just booking a one-way on routes where there are through fares, there are likely very few cases where it will be cheaper than just doing a single through award fare F booking.