Well for the SEA-LAS outbound that was booked under the 2013 SEA Double MQM's promo, I just breezed through the T&C and it doesn't appear that you have to fly a RT, just a segment to or from LAS from SEA. The terms are here:
http://dmn.delta.com/ace_plrs/seattl...es/terms.shtml
The 2014 promo which includes All non stops into our out of SEA is below, it doesn't seem like a one way from MSP-SEA wouldn't work out, as long as I book it before I fly and after i register for the promo (I did that on day one).
http://dmn.delta.com/ace_plrs/seattledouble/terms.shtml