Hi Maureen , I am double checking what ships list you have read , as Moreton Bay / Brisbane was still part of NSW in 1855, I have viewed many records , mostly assisted immigrants from Scotland. And as well as the ships list of Passengers, there is a full page of the individual immigrant , this normally gives the mothers maiden name as well as whether Elizabeth Sophia's parents were still alive.
I have two MC's from the UK in 1854 , no "deceased" mentioned , and I have not been able to establish if they were or not.
Margaret Caley Leng called her first child Elizabeth Golding Leng , why Golding ? There was a family of Goldings on the IOM at the time , but could find no connection.
One reason why I think Margaret Bridson might be Elizabeth's mother is that the Manx usually followed the pattern of naming the first born daughter after the mother's mother , i.e. it gives John and Elizabeth Bridson a good chance of being Elizabeth S's grandparents - we just have to prove it. There are some excellent posters here with a lot of information on various families.
You have not found Sophia on the 1851 census ?

Cheers
David