Yep. Sometimes the 3rd Imperium is very much a mafia. I didn't meant to imply that imperial nobles, or those with the chutzpah to wage a trade war without targeting civilian populations beyond whats necessary for a trade war, wasn't tolerated or in some cases, encouraged. 3I does have a darkside.
Every system of rule-by-nobles has a tendency to look like the mafia.
And a sector or subsector duke within his assigned turf, he's definitely NOT "just another diplomat with a title"... he's the man who can call the navy and cut your world off. In fact, I'd expect part of the 3I membership is granting all "Working" nobility (Planetary barons, viscounts, marquis, and counts) full and locally-irrevocable diplomatic immunity.
Likewise, on-world fiefs are likely to be just as extraterritorial as starports.... the 3I is not, after all, the UK-in-space.