A paid ticket will price out no matter how awful the routing unless it's too complicated for the auto-pricing system.

For a DFW-SLC-MSY-DFW paid ticket, your fare will be a circle trip: DFW-SLC, SLC-MSY and MSY-DFW.

In order to get a free stopover on an award ticket, the stopover must be on the routing. SLC is not on the routing for DFW-MSY and MSY is not on the routing for DFW-SLC.

That is, if you could book a connection in a city on a simple round trip as a paid ticket, then you can book an award ticket with that city as a stopover. If a paid ticket with the desired connection prices as a circle trip even though you are within the four hour layover limit, it means you cannot use that as a stopover on an award ticket.

SLC is on the routing for DFW-EWR because EWR is far enough away from DFW to justify a SLC connection. Generally speaking, the further apart the cities of a city pair, the further out of your way you can make the routing. For example, on NW you can connect in CWA on the way from the East Coast to the West Coast but not if you are flying from the East Coast to MSP, even though the schedule allows DTW-CWA-MSP in either case.