the will was proved in a manx court by son G W Dumbell - widow Ann dumbell als davies can I think be found in Athol street in 1841.
However Johnathan is buried Douglas St Georges 18270713 - the petn by G W Dumbell to the will states died some time ago, probably the Manx decree was need to sort some Manx property - thus Johnathan can't be the father.
Now it is possible that G W had an illicit affair (tho he married Margt Gibson in 1831) and used an alias - however my own suspicion is that as the baptism was recorded in parish of grandparents (who I suspect raised the child) you are looking at a 'humbler' Dumbell family - the name is reasonably common in the Liverpool/Warrington area as I recall from my youth.