Brian made several typos in his index, tho due to the method of construction the error rate is much better than the burials - I suggest you database all the arch wills then check for sequential numbers + sequence of parishes, duplicate numbers then stand out and you can usually guage the correct year - I once suggested to Brian he could do similar internal consistency checks but got a brush off response.
Re Episcopal - the new 'indices' at Manx museum are more accurate but useless in the format presented in that they give a frame by frame sequence - whereas this can be used in a computer search it is near useless for manual search down several pages of names. Some of the episcopal wills would appear to have been dropped on the floor prior to filming and then picked up with quite strange ordering (tho I suspect the shuffle dates from 19th C) - the Museum new indices usually comment on which books/years seem out of order