It's tough to prevent NT coaches misusing it. Being a NT coach has always required a lot of integrity. Especially when you can call up all the players in your country to see their stats. I guess the only way is to vote him out the next election.
That said, since it's a fraction of your salary (and that percentage increases with your NT rank), the NT coach will still have to ensure his player has enough salary to benefit from it while making sure he wins games as well.
That said, I think the idea of top NT players receiving a higher percentage of jersey sales compared to a permanent benchwarmer certainly makes sense. I kinda agree that both players shouldn't have the same amount of jersey sales.