I believe that the answers are yes and yes. It does put you on the standby list, but could exclude you from EUA. I think there are some posts on the NWA board about this. Also, the NWA EUA seems to run exactly 1/3/5 days before flight, not at midnight.

I think the best bet is to wait until after midnight CST to use online check in. You can also check ITN. If there are no F seats available at midnight, NWA probably will not release any before the flight. Seats will be held in revenue inventory until flight time and upgrades will be cleared by status at the gate.