Criminal branch offices have a closing cooldown, so you need to open up a bunch of them and you'll only lose one at a time. I'm pretty sure it's the office with the least crime, so one thing you can do is open up a "dummy" office that the AI will target first. I think there was a patch a while ago that made the AI more likely to accept crime lord deals, too.
One thing I saw suggested a while ago is to play pacifist, and still build up a huge navy. You can't declare war on empires that have your offices on them, but they'll hate you enough that they'll declare war on you anyway, so you can sit and wait while taking advantage of pacifist bonuses. Don't target too broadly, you don't want everyone ganging up on you at once.
You'll want to avoid leaving branch offices on your vassals, as it makes them produce less, which means you tax less. Unless you have a big vassal that you might want to split up through instability, maybe? I haven't tried that.
Viability is a moot point anyway in a game with variable difficulty. You'll have trouble on GA 25x crisis without some specific builds, but imo the fun part of the game is trying new things, getting crushed, and learning.