Hello Sue, and thanks for that.
That John Kerruish on the Eagle passenger list looks like a good match. The age is about right for the JK who died in Sandhurst. Ancestry's transcription shows his departure point as Liverpool, which I assume was a likely departure point for IOM emigrants.
If that is the same JK, it looks as if he and TK Maltby did not travel together as hypothesised. I can find no Maltby or similar name on the passenger list for that voyage, though the Ancestry transcription has a "Thomas" with no surname or birth year. I plan to visit Melbourne next month and will try to see the microfilm.
The marriage and death dates you list are those of Thomas Karran Maltby (though he was widowed and married again in 1887). According to his death cert, he had been in the Colony of Victoria for 40 years, which is consistent with his having arrived about the same time as the Eagle passenger Kerruish.