I suspect you will find the research at the Museum even more tiring than you imagine.
It would appear that your published tree is somewhat confused - I have doubts about the longevity of your Generation 7 John Cain - burial at age 98 would have certainly merited an entry in the parish reg - there are 2 John Cains buried in 1738 OS (both 1739 NS being 2 + 20 Mar) - there is only one will but as doesn't give date of death not clear but this is - son robt caine exec, g-child john mylvorrey which doesn't sound like yours

I hope you can locate the bible - the date of printing indicates the earliest possible year the entries were made - why I asked re hand is that if they are all in one hand then indicates copied from some other source.

Manx records were one of the first to be available at Salt lake City and via the microfiche IGI thus many often poorly researched trees date from the 1950's (poorly researched in sense that they had no easy access to land and property records often needed to disambiguate families as well as often overlooking the many missing records)