In regards to nwa.com, CmdrThor is correct. When you are making the reservation, you cannot select exit rows. Complete the ticketing and immediately go back to Manage My Reservations and you will be able to switch. Also make sure your frequent flier number is entered in the reservation - this could be why you can't select Premium seats. I know for DL reservations with a NW worldperks number I have to call to enter the number. I don't think anyone can select disability seats until checkin online, but you can probably get them if you call.
I also had an issue last week when I tried to change my seat on a CRJ. Manage my Reservations showed everything taken except the disability seats. Reservations showed the plane wide open. I finally realized that the flight was ticketed for the wrong day, and I never caught it. My reservation was for Thursday, but I was trying to "buy" the flight for Wednesday. Not sure if I or the travel agent goofed, but my fault for not double checking when the itinerary was e-mailed.