I'm more concerned about the fact, that sinking English ships after I had sunk the Portuguese ship had no effect. Only the first sunk ship seems to cause the post in the ship's log and affect my ownership of lands and title. After that it's purely the relationship points, which are changed.Yes, that sounds about right. Deliberately attacking ships belonging to a nation whom you're supposed to be serving by having a LoM is outright treachery and automatically strips your title and land. Attacking an ally turns the nation wary, which removes your LoM, but only strips land and title if you have a bad reputation. That way you don't get the full penalty if your crime is failing to pay attention to tavern news, specifically random relation changes during FreePlay.
But I don't know, what's possible and what's not, when it comes to plural nations in a sea battle.