Others can comment but assuming you leave Canada for the US before your authorised stay expires and you return to Canada with details of your flight out of Canada to show to CBSA personally I cannot see an issue but then I am not CBSA.
As a visa exempt holding a confirmed flight out then worst case they simply issue you a visit record with a specific date slightly beyond your flight date or they just wave you in, doubtful although no way to predict they refuse entry.
As Canuck suggests would not recommend flagpoling, coming back into Canada, then going to Seattle then coming back into Canada to leave if that was your intention as opposed to visiting Seattle before your overstay then back to Canada for your flight.
Minor point a 12 day overstay is not a couple of days, if we’re a just a couple days overstay whilst still technically an overstay it is not the same.