I think all national pool federations are members (some indirectly) of the WPA and are governed by WPA decisions.I'm sorry I don't follow the logic. Do you mean he risks being banned from most Finnish tournaments if he signs up with Matchroom? Why would he be banned for that? Is it because WPA can decide to ban, and Finnish tournaments are under WPA..?
I think the WPA ban will apply to any player who plays in an international event (by some definition that includes all or nearly all of the large Matchroom events) that does not have a WPA sanction. I believe the ban starts being applied to events in March.