Liz
I have never employed a researcher but seems a lot of money £220 minimum with no guarantee they will find anything and then do you have to pay for the certificates on top of that?
I still think there are avenues for you to explore but then again you must be getting pretty exasperated by now so maybe a set of fresh eyes and experience may come up with something?
I noticed a Cain (will have to recheck but think a Robert) in the workhouse similar age to the one you were looking for but he was a fisherman so discounted him, is that the reference he made to the workhouse? Or is he referring that your Father was born there? Do you know if there are records of the workhouse available? I can honestly say I have spent a long time looking up your Cains for you on records available via the web and as you know we havent come to any definite conclusions - will the researcher be able to do so? Quite a few others I know have been looking for you and havent found them and I wonder what other records would be available to the researcher that you cant look up yourself over there in the Isle of Man.
Starting from the beginning, you know for definite that your paternal Grandmother was Catherine Cain - do you know for sure she was the daughter of Julia (Without checking I think you do as you made the connection with her sister Harriet??) You have been told that Catherine went to Australia? Maybe she married before she went and therefore went under a different name? We couldnt find a Catherine Cain nor a Harriet on the emigration sites to Australia - maybe she went to another country? Have you checked for any marriages of a Catherine Cain c1920's onwards? Are there any deaths of a Catherine Cain after your Dad was born? Maybe she didnt emigrate, she may have died or married and stayed in the Isle of Man and her new husband wouldnt take her son on?
Going on to John Henry Cain he may be the one with the Dukes but he may not be. His birth wasnt registered so will the researcher be able to find him or come up with a 'conclusion' he may be the one we think he is. Can you check Church baptisms for Braddan for John Henry, Father Robert, maybe he was baptised but it isnt on the FamilySearch site - I have come across this happening before with a few of mine.
Liz, think it will be a case of you pay your money you take your chance, the researcher may just find something but before you ask him to do it for you maybe just check those few things up, Workhouse Records (are they in the Museum?) Catherine Cain, a marriage or a death and baptism records for Churches in Braddan for John Henry.
As I say, I am no expert and just passing on what I hope are a few useful pointers in a last ditch effort you may be able to try??
Sandra