No, you can only place major IC’s on originally owned territory of that power. So Germany could place is on norway since they originally owned it but no other power could since…well it’s Germany’s original territory.
FIC, if under united UK economy, can be useful for UK. I agree with you that it serves better for Japan but once again, it depends on what Japan attacks first.
I recall a game or two of P40 where the DEI were up for grabs. Potentially we had not read the rules correctly or it was changed in the erratica. Either way Japan doesnt seem able to touch them now. Probably a representation of Dutch VIPs continuing the war from England after Germany’s victory