Get in touch with your closing attorney ASAP and have them contact Wells. Honestly the sellers aren't thinking clearly, as working with you and extending a week is certainly going to be quicker than going back on the market.
I would also suggest you contact the NC Banking commission and the Sate Attorney General's office and that you file a formal complaint against Wells Fargo.
Other buyers reading this should take note this is actually a common problem with Wells Fargo, Suntrust, BOA, Citibank etc. These lenders should never be used they haven't closed a single transaction that I've had the misfortune to be involved with on time in over 5 years and I never recommend any of the big banks to any of my clients.