Just posting a mini datapoint on D1 Lounge Access in SEA:
Yesterday my husband flew over to Thailand to join me on my current trip here. For the SEA-ICN segment he was on KE confirmed in Prestige, their business class cabin.
He arrived at the new club entry around 0730, an hour prior to the D1 lounge opening. He used the kiosk to check-in (despite me advising him to go to the desk). The kiosk granted him entry to the club but made no mention of D1 lounge. He asked an attendant standing off to the side if the D1 lounge still opened at 0830, and if he needed to do any other check-in or just go up when it opened. She replied that it did, but he must be flying D1. He replied that he was on Korean Air in Prestige. She said “sorry, only D1 passengers are allowed.”
Now, I had warned my husband (based off this thread) that agents at SEA might not be fully informed on access, so he politely but firmly pushed back. He mentioned that Korean Air was a partner, per the website, that qualified for the D1 lounge if you were in Prestige. Again she denied him saying “you are mistaken - you must be flying D1 internationally and no exceptions to this are made.”
Well, my husband then asked to speak to someone else and at that point the desk attendant apparently overhead the conversation and came over. She immediately apologized and corrected the first agent that Korean Air qualified and had him come over to the desk. She scanned his BP, and politely told him in the future to please come directly to the desk when accessing the D1 lounge as now she had to “undo” his check-in on the kiosk. Whatever that meant. She gave him a white slip to show at entry - not sure if that was because of the partner flight or him being there before it opened.
Once the lounge opened he was the first one in and thereafter enjoyed his experience throughly.
Bottomline, it appears SEA has some issues with knowing what qualifies for lounge entry. I encourage anyone with any access other than a straightforward D1 ticket to be prepared for potential pushback and to escalate if needed.