I don't like the concept of having one player being able to see all information from all players in a country.
If you have a site you could easily farm the NT credentials and harvest the information.
Sounds like you should change your access key more frequently if you're worried about this.
And I agree with you on one thing: giving NT coaches the ability to see players makes it very hard for those users to use another country's bbapi applications safely, since the other country could see their NT roster (not every player from the country, as I said above, there's no way this can be done fairly in the bbapi, but they could see the whole NT roster if the coach is given access to that). Probably the surest solution is to strongly encourage NT coaches to not use any bbapi sites that they don't trust.
Alternatively, a more complicated solution would be allow NT coaches access to dedicated "country" accounts. So, whoever is elected NT coach for the US would become the owner of the USA_NT account (or similarly, USA_U21) and would be able to set the access key for that account. This would allow them to keep their team and NT information separate. This seems like a lot of work though.