And here are why your ideas won't work:
"1. Get the translators to come to your studio to do the translation and don't let them in or out with any media or recording equipment. I believe Apple had this approach when previewing the Watch to some devs."
They'll refuse to put down the travel expenses because it wouldn't be worth it for them. It's MUCH easier and less expensive to send a disc or hard drive than a translation team. If they're THAT paranoid, they can courier the copy with an agent from THEIR studio, with all the expenses that implies.
"2. Remove all the bits where nobody is speaking before sending it for translation. It would probably make the movie unwatchable."
Don't forget signs and other visual translations, at which point it would probably become barely watchable and worth a pirate's time.
"3. Obscure a significant portion of the image with a big black rectangle. Again, it would make it unwatchable."
It also removes key context needed for some translating to make sense. Recall that English isn't exactly the most precise of languages.
"4. Send each scene to a different translation bureau - chances of them all being dishonest is smaller."
As another poster noted, consistency is essential for a good translation, which means it has to be a single firm throughout the run or else inherent translation variations build up to result in misnterpretation which can occur at key plot points, ruining the experience.