Do they know who they are?
The complaint says they signed their first names on a document but not their last names, so they don't necessarily have their names, but then again it makes it sound like the shliach tried to obscure his identity as well, and that didn't stop the FBI from figuring out who he was. Not to mention the fact that the 4th defendant is listed as First Name Unkown Last Name Unknown (FNU LNU) aka "Yaakov". That guy was much more involved (it makes it seem like he's the head 'thug') though so maybe it's different.
I don't see what the eidim would be arrested for. There's no reason to assume they had any clue about the plans for violent coercion.
But the reports are that six additional, people have been arrested - how do you know that does not include the eidim?
Based on the complaint, the 2 Rabonim and the FNU LNU guy is obvious. Anybody else would have to be some sort of conspiracy charge. I'm not saying they should arrest the eidim, just that I would think from a conspiracy standpoint, there's no difference between the eidim and the shliach. Unless the shliach has to be there while they get the 'rotze ani' out of the husband?
I hadn't thought about the additional 6 mentioned, it's certainly possible. They haven't charged those people though, while they have charged the shliach.