What boxes were checked G-28? Does lawyer receive cards too or not? (there's separate box for that). If yes, you should bug lawyer to help you resolve it. Also, you can try talking to somebody in USPS office which serves lawyer's office.
If card was mailed to your address, go to local post office, show them delivery status and ask the questions.
Also, do you still have 2 year GC? Do you have extension letter which is still valid? If yes, you can travel with those in June if you don't have your GC at that time.
Also, after talking to USPS, if they cannot help, you should consider filing non-delivery with USCIS : https://egov.uscis.gov/e-request/displayNDCForm.do?entryPoint=init&sroPageType=ndc
I understand how stressful it is, but you still have time to resolve it before the trip. I recommend to always wait for green card / passport etc before booking any flights.