Umm,
Two thoughts....
Re Catharine Woods - not sure why she could NOT be in Hobart during those times, as I understood that when General Musters were called that the inhabitants (especially those under convictions) were required to attend with their Masters, thus perhaps Catharine Woods went to the Muster where her Master was reporting at...

... We have
NO Muster with Catharine McNally named on it... 
and so I think Catharine Woods ought to be part of this picture still.... Surely Thomas Ransom would have visited his rural land grants, perchance ....
Also, if Catharine was 20 (could be a transcript error for 29) in 1817, then she would be in the picture for motherhood of Ann and Thomas....
Re John Skelton/Skilton .... That's a good question Robyn... I think the 48th arrived 1817 and 1821, and were in NSW and VDL, but I think this chap would have arrived perhaps earlier

Cheers All,
JM