ARN does have a dedicated security screening fast track. Last I used it, I was flying DY intra-Scandinavia and showed my Platinum Amex card to use it. But I’ve also used it at ARN when flying non-Scandinavian carriers.

I would say a dedicated premium security line doesn’t necessarily mean dedicated to passengers of just one airline.

Amex Platinum works for fast track security for card presenter + one guest at a checkpoint at each of the following Nordic airports/terminals:

Stockholm Arlanda Terminal 5
Stockholm Arlanda Terminal 2
Göteborg Landvetter Airport
Malmö Airport
Helsingfors Vanda

Business class customers flying select SK-code, partner-operated flights have never had access to the SAS fast track at CPH?

At ARN, the SAS fast track used to also be available to some LH customers and was signed for that for a period of time. There I stopped paying attention to the signs some years back.

Only if they have EBG or EBD status. Then they can use the SK fast track for any *A operated flights no matter the class they are flying on.

I've been through that airport + 100 times over the last 3 years and I never paid attention to the sign's, but I'll try to remember to do it next time. :-)

For SK with LH+LX I do remember from years back. In more recent years it’s for a lot more than it used to be, and for Amex Plat/Centurion card holders too. The sign of who can use fast track was illuminated and on the wall closest to the airside part of the terminal when going toward the security checkpoint. The sign has multiple airlines + Amex Something listed last I used it, but I stopped paying (detailed) attention to it some years back.

Yes, sort of. But ARN fast track for SK also changed over time. And CPH fast track for SK may end up following the same course, as that too hasn’t been absolutely static over the last twenty years.