you are probably seeing that connection/combination on the NW website. i have done these before, and i havent had any problems. NW wont give your seat away if booked like this because they know you are coming from QX. the only reason they will give it away is because you are going to misconnect.

that being said, if you are in SEA and it looks like the QX flight is going to be delayed, i would have the AS agent rebook you then and there on a NW SEA-MSP nonstop -- it can be done when there will be a misconnection.

with NW having only one (or maybe two) flights out of GEG, you miss that flight you are going to end up back in SEA anyway.

yes, it will be tight, and you may be one of the last on the plane (and not have a place for your hand baggage), but if it is sold via the NW website and shown as a connecting flight you should be fine.