Weren't you doing some crazy MHT-DTW-SEA-PDX-SEA-DTW-MHT kind of run last weekend? (when we were on the same SEA-DTW flight). I was doing the SEA-DTW-JAX-MSP-SEA MR myself.
I've encountered this same problem on previous MileSafari type programs. I'll end up flying SEA-PDX-NRT-HKG or something like that (usually since NW 7 SEA-NRT is always full/overbooked like 325 days a year). I forget that just the little 140 mile jump from SEA-PDX on Horizon will disqualify my mileage bonusing opportunities sometimes. Even one little CO segment will do the same. One time I was on SEA-DTW-MEX-IAH-MSP-SEA, and one seg was CO and the whole thing got tossed out for the promo. I was pissed!
Perhaps you should just sign-up for a new 1-year WC Membership. (50 points) I hear the shower is pretty nice at the DTW one