What I'm wondering about is dual-nationality can you be a citizen of UCAS and a citizen of England? Corporations supersede national SINS going by the book so you'd be a citizen of Shiawase not a citizen of Shiawase and a citizen of UCAS and you obviously can't be a citizen of Shiawase and a citizen of Saeder Krupp.
For most intents and purposes, governments only recognize two cases: you're either a citizen, or you're not. Governments consider their own citizenship supercede any other status you might have. You must pay taxes, answer military draft, and whatever else citizens are supposed to. And they ignore any other citizenship you might want to wave at them. So a UCAS-British citizen can't require British consular assistance if he's arrested in the UCAS, and could be charged with treason (instead of espionnage as a foreigner would). Some countries do have accords for limited recognition of dual citizenship (the most common topic being recognition of mariage and military services made in the other country).
Most countries actually make it impossible to abandon citizenship, even when you get another one (but they of course do so while allowing themselves to revoke one's citizenship should they want to). As far as I know, the only one who does is the US (making it possible to escape the IRS if you settled abroad).
Interestingly enough, the Vienna Convention forbids appointing as a diplomat a citizen of the host country (unless the host government gives consent, but it can lift it at will).
We are not talking passports, we are talking Diplomatic immunity... That little thing that allows a diplomat to beat someone to death in front of a room full of cops, and they the cops, can't do anything! When Diplomats break the laws, Generally there is nothing that the Foreign Country hosting the Diplomat can do. Sometimes however, that host country themselves will ignore the Immunity and make an arrest anyways. For that to happen, the rime has to be pretty nasty.
Diplomatic immunity is against search, arrest, detention, prosecution and testimony. The Vienna Convention does grant "freedom of movement", "free communication", and require "due respect" and to take "all appropriate steps to prevent any attack on [a diplomat's] person, freedom or dignity". So in your example, the cops can totally grab his arms to hamper his freedom of action, or even, depending on your definition of "due respect", shoot him or use a taser on him. They just can't put him in a cell thereafter. That's certainly going to spark a diplomatic crisis, but that's not a violation of the Vienna Convention.
At least in the real world. Not in Hollywood.