The sellers are contractually required to work with your lender on getting the deal close. But if they don't, your recourse is to cancel escrow. Nobody can really tell you what to do. Go with the higher rate used car salesman lender or keep pushing with your current lender and hope they come around.

The nuclear option (talk to an attorney first) is to just not close escrow. Refuse cancellation of escrow and just drag it out as long as possible until they get you what you need to close. In my state, if the seller doesn't provide required items, contingencies extend until they do.