In defense of my cohorts in Seattle, the UMNR should never have been accepted from Northwest, and the Delta agent in CVG that rerouted the UMNR should never have issued the ticket for that routing, as neither Alaska or Delta will connect UMs between them or with Northwest. Delta will only connect UMNRs to Delta , Delta Connection (non-AA Eagle) and Air France. Alaska will only connect UMNRs to Alaska, Horizon, Pen Air and ERA.
I can, however, state that if a UMNR has to overnight in Seattle with Alaska, the usual process is to take them to a hotel with an agent of the same sex stationed outside the door all night. In all honesty, it's probably the cushiest overtime assignment there is to be had. If, however, it's very late at night and the next flight leaves early, it has happened where kids have spent the night in the UMNR lounge, which is a controlled acxess room, with video cameras watching all parts of the room and with only specifically trained agents assigned to work there. Back when we did accept UMs from other airlines, we always went by the more restricted age rules between the two airlines. If Delta required a 13 year old to travel as a UMNR, then we would require the child to be a UMNR for their Alaska flights, even though at age 13 we will allow them to fly as a Teen No-Assist. The reason for this was primarily the fact that, if a 13 year old showed up alone at a Delta gate or ticket counter without a guardian, they wouldn't accept them at check in.
This is a best guess, but from what has been mentioned already, the UM probably arrived in Seattle on NW 953 which is scheduled to arrive in Seattle at 9:11pm. The only flight left to Fairbanks, n/s, on Alaska would have been flight 125 which departs Seattle at 10:55pm and arrives in Fairbanks at 1:34am the next day. The only occasions where Alaska will board a UM on a flight that late is if it's the only connection available in a certain market. Back when Boston had only the one flight a day, it was not unheard of to see UMs connecting to the last flight of the night up to ANC, as that was the only flight available.
The Alaska agents in Seattle did what is, according to our procedures and rules, correct. Arguably it might not have been the right thing to do, but the cause of the problem was Delta, specifically the agent that rebooked the UMNR onto flights on other airlines when Delta's own website states that they will not connect UMs to flight other than their own, Delta Connection and Air France.