Hi Grasc,
I am not related to the good Captain, however, his daughter, Eliza Ann(e) married my 3 x gt grandfather, Dr James ELLIS RN (the surgeon on the Bengal Merchant) after their arrival in Sydney. It would seem that he was perhaps still married to my 3 x gt grandmother, Elizabeth ELLIS (nee FUZZEY) at the time

After reading your post I found a record to the Australia Death Index 1787-1985 for Robert Stair Graham MACDONALD 12 Sep 1907 in Queensland showing his father as George MACDONALD and mother as Sarah MITCHELL Regn No. 003967. You might already have these details.
After Dr ELLIS died in London in 1863, Eliza Ann(e) went on to marry Dr John STIRLING RN. Despite professional researchers looking into my Dr ELLIS' naval papers (and thirty years'research) I have yet to find out where he was born - I would love to know. One interesting thing I've been told by the family grapevine is that the ELLIS family had some association with Lord GRAHAM after which Grahamstown, South Africa, is named. I don't know what it is though. Dr ELLIS seems to have spent quite a bit of leave at different times in Londonderry so he may be Irish. Seeing the GRAHAM name in your post I am wondering if there is any distant relationship.
If you would like to know more about Eliza Ann(e) I am happy to share it (what little there is).
Kind regards,
Lisa