I am going by memory of your first post and the rules so take this with a grain of salt.

If you are before the 6 hour time frame, see and agent at the ticket counter. Once you check a bag, no one is likely to change your flights, either in SEA or MSP.

If you have no checked baggage, try in SEA to get the standby, by using the ticket counter agent. If not, try at the gate for the MCI flight in MSP.

There is a slight to good chance the agent in SEA will confirm you all the way through. Same day standbys have no charge at NWA, unlike many other airlines.

Good luck.