possibly our resident Cain expert Donna will rise to this - however I'm confused
The first Peter is you say Irish - the name Cain is not unique to IoM tho the Irish variant is often seen as Kane - though Peter is a first name little used in the Cain - so rare I only have 2 between late 17thC and mid 20th in my records.
Then you say all your John Cain's relatives are Manx - maybe if you name those you are sure of it may help in finding a known emigrant - if there are any males in a known direct line it might well be worth getting one dna tested with a detailed test as one main Manx Cain carries DNA variants unique to Viking era males whereas I understand the Irish sept may not