niobium - diamonds is a legal tradetroute. - any attempts to turn it illegal are ooRP and, frankly speaking silly. the reason to do that is to get rid of ooRP traders maybe, but there is no RP base to limit trade of those items.
in RP, this traderoute is a main backbone of the kusari economy between two houses that are politicly close to each other. taxes and licenses would be included in the pricing allready anyway - so attempts to add a toll to it or other stuff wont work. they are not dangerous materials like plutonium and cannot be used to craft weapons and or warmaterial that may turn against one another.
so - if you wish to limit the trade of those materials - pirate the traders, its the only valid method when you stick to RP. niobium is used in shipbuilding in rheinland - and rheinland supports kusari, diamonds are used for optronics and jewelry in kusari. so it benefits kusaris economy, too - cause they export optronics.
i d not try to limit something that is heavily used, just cause its heavily used. limitations must be based on a good RP reason why its limited. - even turning a blind eye on a pirate that is pirating the traders there is ooRP in some sort, even if the player sometimes wishes to do it - but its not in character... a kusari char aswell as a rheinland char would try everything to support that trade rather than limit it.
other than that - i like the approach on these laws. and the status of the KNF is much more appropriate than in other houses. that means they represent a strong and maybe most powerful BRANCH of the KNF, but not the emperor itself.
so there actually IS a backdoor for good RP for independents for each shipclass, in contrast to rheinland that has closed all options for certain ships for non RM members.