How to evaluate this board situation under Japanese or European rules
I will only discuss the Japanese pro rules, since as far as I know there are no official European rules. The tournament of the European Go Congress 2015, for example, was played with AGA rules. I think most European tournaments are played with &quot;Verbal European-Japanese Rules&quot; which usually require to fill all dame points and kos before the end of the game. Then I think the position would be counted as a Seki by the referee.

Senseis claims that these are the official Japanese pro rules in a translation of James Davies.
Two paragraphs are relevant for the question:

9.2 After stopping, the game ends through confirmation and agreement by the two players about the life and death of stones and territory. This is called &quot;the end of the game.&quot;
9.3 If a player requests resumption of a stopped game, his opponent must oblige and has the right to play first.

and

13.1 After the game stops according to Article 9, if the players find an effective move, which would affect the result of the game, and therefore cannot agree to end the game, both players lose.


As you already noted the best B can get is a Seki and the best W can get is killing the black stones. Therefore the only interesting situation is when B wins in case of the Seki and W wins if W can kill. Then it is important which player has more ko threats, though most likely the kos where already played as the situation appeared after both players passed.
My interpretation is the following:

B has more ko threats: B claims resumption according to Article 9.3 and W plays first. W can play the ko but cannot win it. B wins.
W has more ko threats or no ko threats: B does not want to resume the match since B will lose the ko. In the case of no ko threats W takes first and B looses the ko as well. W does not want to resume the match since it would be B's turn and B will complete the Seki which results in a loss of W. Thus both players lose according to Article 13.1

On Senseis you can also find commentaries on these rules pointing out some ambiguities around these Articles and adding interpretation.
Edit: One of the comments there is this one which I think clarifies that the position in the question would result in a loss of both players. The relevant paragraph is §13 - Interpretation of the Rules of Play. The example there is similar to the position in the question. The point is that both players will agree that there is an effective move. Therefore Article 13.1 is to be used. W cannot claim that the stones are dead as long as this effective move exists. It is not important that this move will not be played as no player will ask for resumption.