A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 100,417
Look at your employer's travel policy. You might be required or expected to use SAS, for example, from CPH.
I don't think there are DL partner or SkyTeam nonstops from CPH to LHR, so connecting onto VS for TATL travel is unlikely to work well and/or be priced reasonably, but YMMV.
What will be your typical routes in addition to travel to SEA? On SkyTeam, your realistic options will be KLM through AMS (probably better) or AF through CDG. It sounds roughly like you'll probably qualify for DM (top published tier at 125,000 status miles) status with DL which as SkyTeam ElitePlus (GM/PM/DM on DL with corresponding FB tiers) gives access to all SkyTeam partner lounges on international itineraries, but DL is stingy about giving access to third party lounges, including at some airports where AF/KLM do make such arrangements.