SImilar situation as OP
I bought a ticket today in "Premium Select", SEA-MSP-AMS-KBP. SEA-MSP in DL Comfort +, MSP-AMS in DL Premium Select, and then it is a bit of a mystery what I have from AMS-KBP (on KLM metal but a DL flight number). If I look at "my trips" on Delta, it is listed as "First Class". But the seat map is not letting me choose anything in F. I don't much care as intra-European "first" is nothing special, but I am curious as to what I have. Fare codes are W for SEA-MSP, G for MSP-AMS, and G for AMS-KBP.
I am not a studious follower of MQM rules, but I was pleasantly surprised to see that Premium Select earns a 50% bonus on MQMs, same as DL One, even though the fare on Premium Select is far, far less.
By the way, will Delta ever introduce Premium Select on SEA-CDG or SEA-AMS flights? If yes, what's with the delay as we're now, what, two years into the introduction of Premium Select? If not, why not? I am connecting in MSP on this itinerary pretty much for the sole reason of wanting to try it out.