Monaco

This might just be a shot in dark but as Nalan is a SkyBreaker (THE skybreaker) his one big ideal is that no law should ever be broken, whether it be the law of that particular land or people. It is often mentioned that the Spren that come back to bond with the new radiants are breaking some form of law in the cognitive realm by doing so. Is this the reason then? Is he doling justice to the spren first and the 'law breakers' second?

This might be an odd mix, but I would go for Scadrial + Nalthis, specifically Feruchemy from Scadrial, but only if I was going to be on Roshar. That way I could use nicrosil metalminds to store the investiture I could gather during a highstorm (as a replacement for Breath which I'm guessing is possible). Then I could create an army of helpers and awakened items to do my bidding, have all the benefits of the Heightenings, and still be able to use a full set of feruchemical abilites