It would behoove you to have an Metrics validation review monthly to review the data submitted, review the TL alerts if any, and review the outliers in the data. If there are outliers, those can be traced down and examined. It helps to have a second or third set of eyes review the data calculations periodically (confusing things they are!) to see if methods of calculation have changed or if the process of data collection has changed.
As far as relying on a CB auditor to catch errors, I do not hold faith that they are able to catch some or all errors given the limited amount of time they have to examine the calculations, retrieve the data, do comparison analysis, etc. If they do happen to catch an error, great! But I feel that it falls to the company to review methods and calculations and validate the data prior to submission. JMHO.