Per a post from azj over on A.net, the Northwest callsign will still be around until reservations cutover, at which point everything NW (both customer facing and operationally) will fade. Starting on Jan. 1 though with SOC, pilots can be interchanged (and I believe that the first mixed DL/NW crews will be in February/March).
As you mention, the FA situation won't be done until a vote, which won't occur for a while (until the NMB figures out if they can change the rules or not).
With Airport C/S, I know at MSP check-in, they've had both legacy DL and NW employees at the Medallion counter - they'll often pull over a legacy DL agent when I've got some complicated DL issue (I seem to get a lot of airport reissues...) but it gets figured out in the end.