It is really just as easy as asking. It is not in NW' interest to fly you from hub-to-hub to destination. The good thing is that you'll only have carry on. Just get to the airport a few minutes early and ask the agents at the ticket counter. If they can't confirm you there just make sure you have all the info you need, gate info, departure times flight numbers etc... You shouldn't have much of a problem if there is a seat open on the flight.

Your MCO-SEA trip: Once you get to MEM just try and get on the 8:40am flight to SEA (Flt 877) You'll be home before noon... but might have to sit in coach. Of course upgrades with regards to standby is a different beast, it just goes on space available.