Hello Shane, I think you have missed a point. Sue correctly surmised in her message on the 19th that James Leece is your Gt Gt grandfather's half brother. The second eldest of James's four children was named Thomas William Davidson Leece.
I do not know the christian names of T.M. Davidson and therefore cannot state his relationship to James Leece..
The occupation "servant" would relate to his occupation in the IOM. He is a relative and not James's servant.

The following may have more relevance to you, than it has for me.
"Immigrants whose passage money has been paid in the Colony - - New Plymouth 49 - Leece -Jas."

"Taranaki passage orders granted for nominated immigrants under Regulations of 1st March 1872.
No.49. - J. Leese, wife and 3 children {the expected third child had a twin] 36 yrs Labourer, Crassacally
[Croit-e-Caley] near Castleton Isle of Man."

"How finally disposed of --J. Dumbell, New Hampshire "

Returning to Sue's message of the 19th. Her assumption that John Dumbell kept in touch with his mother and her children and seemingly with his Davidson relations is borne out by the fact that the passage money had been paid in the Colony.

John Dumbell was, in 1874, the only near relation to the Leece family, living in N.Z.
Harry