airbags, module and sensors replaced, dealer can't get light off

ONLY way to determine if there is a problem in the harness is to replace KNOWN failed(or deployed , as in your case) components and re-test. Quite likely scenario , and I wouldn't want to be in either one of your positions(yours or shops!). Ask the shop for ability to CONFIRM and guarantee(within reason, I say , Not knowing the actual extent of damage) the harness , as actual repair to turn light off. Insurance not involved I guess? Is it to late?