I think instead of creating a new system, revising the current is best and making your case for why it isn't a rule violation is better than complicating the current system.
Lets look at whats wrong with your revised idea now,
The reason this 25fame thing exist is to protect people. By adding this new suggestion, you take away the protection this person has. "keeping your fame low intentionally, and not arming trade fleets are also in a way a cheat of the system with the characteristic of high amounts of ships." Well, in that case, "intentionally lost a battle to keep Capt D above 25 fame" is also cheating the system. What Capt.D did is less of cheating the system but making use of the built in protection. It isn't like he changed his name multiple times to get lower fame *cough arkadien cough* or he purposely lost battles to lose fame. What this suggestion is doing is basically overruling the current protection system which might be a pain for you the aggressor, but for the person being attacked, it's exactly what's needed to keep the game fair.
I stand in strong negation of both presented suggestions.
-Roi