There's nothing to sue over. This was laid out in this previous episode where they had the meeting with Tim in Helen's office. Go rewatch it. The only prospective lawsuit was if they went forward with the show without Tim getting anything. He demanded that he be a part of the show or he'd sue. Sean said fine, Tim can run the show, but there won't be a show for long because Sean and Beverly wouldn't be involved. Tim has no rights to sue over Sean quitting the show since there is no intellectual property infringement when Tim is given the writer credit, it's just a failed show. This is why he was told, directly, that he could run a show, but it won't exist for long. They all accepted this would be the reality if Sean quit.

This was enough to make everyone back down, and Helen wasn't backing up anything, there wasn't anything she could do about it. She and her network wanted the show, so if Sean and Beverly were legally unable to quit the show somehow, that card would have already been pulled on them at the time. They're free and clear to walk on it, all they've sold is the rights to the initial script, they're not obligated to write more.

The network has a script for one pilot, not for an entire series that hasn't even been cast yet.

So again, it is internally illogical for the show to treat them like Helen's trapped in the show with Tim when A) Beverly had to be dragged kicking and screaming to do the show in the first place and isn't that committed to it, and B) Sean already gave an ultimatum about quitting under these exact conditions not that long ago. No, they can't get past Helen's insanity and still have their show the way they want it, but both have shown a willingness to walk on the show if it was going to be anything like Pucks or worse. That's the entire running theme on the creation of the "opposite of us," that it had to be a different experience or else they wanted nothing to do with it. Entire episodes were dedicated to this subject.