Forced to work as a postdoc without pay
For some reason there is this belief in academia that people should work without pay. It's inappropriate. You need to walk away.  

Even early career researchers are allowed to work for pay.  

It would also seem that a 6-month grant would only be for a rather small project. Can the project be tied off quickly so that you can move on? 

If the senior PIs are threatening to ruin your career, I would try to get some evidence of these threats. Also keep in mind that even if you finish the project, PIs who make threats to ruin you career are not all of the sudden going to become your best buddies. You could very well finish the project and still have your senior PIs blacklist you for further grants. You owe them nothing.