Nice theory,
I agree that if the relationship did originate in 1914, and given the good character and esteem both McNally and Ransom seem to have held within the very small confines of the Hobart town community I would like to suggest the following
(probably not proveable

)
Ransom travelled from NI to PJ in May 1814. all agree?
A son was born to Catharine and John Macanalty
V1811233 5/1811 and V18112491 1A/1811 NSW BDM
Perhaps Ransom was a lodger in their household (he had to have been staying somewhere). Most homes took paying guests to supplement their meagre incomes.
We all know how domestic violence is a tragic part of modern life and I am sure it was the same in 1814.
Ransom may have been a knight in shining armour and rescued McNally from a violent relationship, perhaps even assaulting John Macanulty/McNally. Their initial realtionship may mereley have been that of victim and protector.
There may well have been only a public verbal altercation, which would be enough to set prying eyes and ears and wagging tongues on full alert. Society would have been quick to condemn and flight by both to Hobart a real escape in more ways than one.
This theory offers reasons for
The delay between leaving NI and arriving Hobart.
The obstacle (the marriage to Macanalty/McNally).
The development of the relationship between the two.
The esteem accorded to both individuals.
Christina's absences at musters (she would have been Manalty prior to 1814, and may have given assumed names or names of deceased females at other musters to prevent her husband finding her. (obviously her wherebouts did not reach his ears, most likley because public sympathy was with his wife)
Doesn't rule out Wiggy's latest suggestion in respect of her parentage. Her birth and marriage may well be in the regimental records.
Even the newspaper reports in Hobart which were reasonably sypmathetic seem to refer to her a Mrs Ransom.
Nice little theory but no proof, unless a report can be found in court recors or newspapers of that period.

Robyn