Hi there, Leo,
Like you, I am not anywhere near Tasmania. In fact, I live around 1700 kms from there. I am not related to anyone mentioned in the three Wiggy threads, but I am not currently still looking for anyone mentioned in those threads.
I do note that on a submitted tree, there's mention that Catharine was twice married, firstly in 1816 to a Thomas Jnr Ransom, and then in 1830 to Frederick. Catharine's relationship to both her children (Ann and Thomas) is noted on that tree as "step-children". Ann's birth year is noted there as 1817, and Thomas as 1821.
In my notes I have some details about their possible identities and I will post that information shortly, simply by modifying this post.
MODIFYING:
By Dec 1820, Thomas Ransom was operating Joiners Arms ... there was a coroners inquest there in early Dec 1820 on a William Cobb who left a wife and two children "wholly unprovided for". mentioned in the newspapers about 9 Dec 1820.
Short version of long story. Cobb was convict, his UX was Judith?Johanna/Hanna Conway (Judy Cowy on 1820 VDL muster).. She married again, 1823, 2nd husband was a James LOWE ... stayed in VDL ... but there's no sighting of the two orphaned children, OR of her third child, who was baptised in May 1821.
Possible Scenerio
Judith Conway PJ to VDL late 1816 ish, meets Cobb say 1817, and has a daughter and/or a son by him. perhaps Ann in 1818, and Thomas earlier than Nov 1820, perhaps actually Nov 1819. William Cobb has accident, passes, distraught widow gives up her two children to Thomas Ransom as she is already carrying another child by Cobb. Child was a boy, and baptised with the latinised name for William COBB ... Guielli etc. Rev'd Knopwood baptises C C Mc N's "son" (ie Cobb's son by Judith Conway) because that's part of his responsibilities, and knows full well that Thomas Ransom and C C Mc N will care for both the two children cause perhaps Thomas Ransom followed up and organised the whip around to provide Judith Conway with accomodation, food clothing etc
I have NOT found the three children that Judy Conway had with William Cobb (some submitted trees list the youngest, but not the others) ... TWO VDL children born same era as Ann Ransom and Thomas McNally.
PS, by mid 1822 the population of VDL was only around 8000, (HTG Saturday 8 June 1822) so two orphaned children in Hobart Town should be easier to find than C C McN ...on the 1822 Muster ... surely.
Cheers, JM