Is she blaming you after 14 years? What kind of warranty do folks expect? Whatever you do is going to be a mess, taking the roof off and old insulation off, or cutting new blue board to fit between the joists. Could the old, drooping sheeting be glued back into place with spray adhesive? Maybe staple screening over the joists to hold it up and camouflage it?
I agree the best way would be to take the roofing off a couple of pieces at a time and put new insulation under. New screws would be in order, with a tiny squirt of high premium caulk in each hole. Even the screw gauge might need to be a size larger to use in existing holes. I have taken screws out and then put them back, only to not hold. It would take longer than roofing a new barn.