From the perspective of what can be done about the past, the answer is nothing.
D8 did nothing wrong. OP booked and was checked in for their flight. It presumably left ontime (as OP missed it). Thus, no EC 261/2004 compensation of any kind, including duty of care (hotel + meals).
Having missed his D8 flight, OP is stuck with his ticket's fare rules. Those would generally provide for no value for a no show. However, it is unclear to me that D8 was not willing to rebook (but not for 2 days). Thus, OP purchased BA tickets for the next day.
Many countries treat dual nationals as solely their citizens for immigration purposes. The US for example, requires dual nationals to enter the US on their US passport (although there is no exit passport control).
OP's sole remedy is to make a claim with the Finnish government. I can only imagine just how far that will go.