Well, this is a funny situation that USCIS themselves ignore their own law.
- You should NOT work beyond a total of 6 years even though your extension has been approved by USCIS. USCIS may have approved in this in error. Yes, they do make mistakes!
- Since your I-140 has not been approved, you cannot really prove your case in future. USCIS generally will not consider their own judgment mistakes as their problem. The onus will be on you to follow those laws strictly.
Our advice is to leave the US on 6 year completion unless you get your I-140 (green card application) approved or in process before 6 year mark.