The OP is offering a paid for service, and obviously can't do it, nothing condersending about that, just a lot of concern that the OP is in the wrong line of work. being a full time bike mechanic requires you to to be able to diagnose and fix all type of bikes, in this case, the OP hasn't been able to fully diagnose the issue, for STI's being fixable, you don't know if it is possibe until the issue has been diagnosed, and there is the rub.