How many options do you actually need to fly CPH-SEA? I assume there's only one daily LHR-SEA flight on BA (maybe 2 in summer?), but are there really more varied options on SkyTeam/VS? Or are they all around the same time anyway.
I don't know much about AS's program or its partnership with BA, but generally AS miles are considered quite valuable, no? (DL is called SkyPesos for a reason!) So maybe it's worth putting up with BA? To the US East coast there should be multiple options (depending upon what city) on BA. I guess the problem is intra-Europe, you're not going to want to fly CPH-LHR, but then frankly flying via AMS/CDG isn't a
whole lot better. For those short hops you might be better off on Norwegian (if they're still around) or SAS or whatever's most convenient.
Lastly, on
#3 , I'd just note that elite status benefits are limited on business class flights, basically just the elite mileage bonuses. And if these are pricey tickets in the first place, DL caps the earnings at 75k per ticket (so if you pay more than $9k as a DL Gold, for example, your miles earned are constrained anyway). Of course if your company is buying expensive tickets, then you'll accrue miles rather quickly and SkyPesos may not be such a bad thing.