In reply to your points Leanne:
The rootschat thread you reference was 2008-2013. In the thread, Lorraine Egan could only speculate about a possible connection to Patrick Cussen and Ellen Jervois, the baptism and Chancery Court records had not yet come to light. Jenny found them only last week.
Lorraine also pointed out the Jervois/Hungerford connections as an explanation for "Ellen Hungerford" appearing on the immigration record. One of SJC's maternal Hungerford relatives, Capt. Emanuel Hungerford, emigrated to New South South Wales in 1828 with at least 7 sons and a daughter. If any of these people were known to the record clerk, a conversation about family connections could easily have arisen which led to an incorrect entry.
I have notified descendants of the recent record finds, including Neil Smith who manages the wikitree profile for SJC, and I see he has already referenced them on the profile. I have no doubt that as the new information comes to the attention of descendants, we will begin to see trees amended. Incidentally, Neil mentions that the informant for SJC's 1894 death record was his grandson William Cussen Cormie. (So not at all surprising that errors could be made about the names of his great-grandparents).
I have not referenced DNA matches in this discussion (or on SJC's profile), because although they point in the right direction, the findings are inconclusive at present. But for any interested researchers:
My father is a well documented descendant of Samuel Jervois b.c.1770, brother of Ellen Jervois who married Patrick Cussen. He has 3rd-to-distant cousin matches with three descendants of Samuel Jarvis Cussin, each though a different child. This triangulates the match firmly to either Samuel Jarvis Cussen or his wife Jane Sullivan. As anyone familiar with genetic genealogy knows, things can only be ruled in, not out. So if/when a match can be triangulated with just one other descendant of the Jervois family, it will be conclusive evidence of Jervois descent for SJC. (The coupling of Samuel+Jarvis as names already points us there).