1) Yes. You can do this. An agency will never tell you to skirt the fare rules and do a point beyond ticket. If the agency gets a debit memo down the road, they will bill you for it. The PROBABILITY of this happening is slim, but it is certainly a POSSIBILITY. No checked baggage, of course.

2) They may want to see a ticket leaving Japan.

3) This is true if it is all in the same PNR. If you are booked SEA-NRT-ICN-NRT-SEA, and you no show for the NRT-ICN, the downline itinerary will most likely be canceled by the system. A separate itinerary will probably avoid this with UA, and definitely with another carrier.