Regarding the seat assignment I have no clue how the Danish agent got the seats and how to "override". When I worked for SAS I can assure you that the sales agent has no possibility to "override" and get seats that are occupied. In that case they have to call the airport in question where the flight starts and ask them wich is not always easy, it cannot be done through the booking-system..of that I am 100% sure.

I have no clue how they did or if the agent in TLL just missed some seats?

I personally understand and agree with you on many points, the agent behaviour was not acceptable at all.