They won't take pseudoswede. I can tell you now that entering/exiting the EU through ARN is either very easy or very difficult with no in-between. I was trying to leave the EU through ARN (ARN-JFK on Delta) and was in the "All other passports" line to be stamped out. While I was in line, they had a shift change and flipped the line I was in to EU/EEA passports only. The other people in line saw this and someone mentioned "what are they going to do, refuse to let us leave?" Then when the next person went up to the counter, they were told that they could not be processed because the line is now an EU/EEA passports only line. The person protested that they were in line for a long time and that it's not their fault that they swapped the lane. The Swedish border guard explained that they did a shift change and that the previous guard took the stamp with them, and so the current guard could not stamp anyone out of the EU; so that's why the lane was EU/EEA passports only.

So, we all got into another line as my flight was just beyond the counter, with a final boarding call. Of course, I had the dreaded SSSS on my boarding pass, so I had to go get my bags and shoes swabbed, but the person doing the check took my boarding pass to the gate agent to scan, so that they wouldn't leave without me and could load my checked bag. Thankfully, the process was quick, I wasn't the last person to board my flight, and even after boarding completed, we still somehow pushed back 15 minutes early.

A few days prior, I had to get stamped out as I was flying ARN-LHR on SAS. Absolutely no issue, and I was quickly stamped out with no line for the desk.