The conscious dropping of Manx language began I think post revestment when any jobs (mostly off Island) needed English

Leslie may well be corect in his surmise - certainly in the early registers one sometimes sees names such as ylchreest which never did drop the Mac thus possibly indicating a conscious decision - however somewhat strange that all vicars/curates did so at about the same time - and certainly not all Vicars were 'English stooges'

Most other records also start around 1580 though Lib Placit I think has material from the century before though how much I don't know
In the list of names of the Keys (though there appears a gap for the key years 1507-1585) and the Deemsters the mac seems to be dying out by c.1550