Well, at least one thing hasn't been tried - form 3911. https://www.irs.gov/pub/irs-pdf/f3911.pdf
This will put a trace on the refund. I have had, on more than one occasion, returns that were assigned to someone who either was moved to another location, resigned, or died. In every instance, the returns were lost including 12 of one client's 941's.
I'm assuming that you got the transcripts confirming the refund. At least you can show these to the client so they don't think you are doing nothing.
One thing I would have done after the second stall, is to ask to speak to their supervisor. The supe of the guy who said he couldn't do anything might have been able to do something as they have access to more info, and have much more experience than the phone people.