I agree with all the suggestions made by Sue and Jean (JC). I think Jane will fall in the gap between Cathe (Jan 1810) and Christian (Feb 1815), so I'd concentrate on mid to late 1812 as the most likely. Cathe was St George and Christian was Braddan, so the family changed parishes - Jane could be in either one. I've done every trick I could think of to see if she's in the IGI, but no luck.

Nelson and Robert both bapt the same day 1800 could be twins. I think Nelson will be the burial at Malew in 1824 and Robert lived to be 82 and made 5 census's in Malew.

This leads me to think that the whole family moved to Castletown or Malew between 1817 (last child baptized) and 1824. The burial of a Paul Fell 27 Aug 1832 in Malew may be the father though there are other Paul Fell's.

The eldest son Paul William b 1792 may be the Paul FELL age 58 blacksmith in the 1851 census at Liverpool. There is Jane Fell 42, presumably his wife. They are lodgers.

I didn't get anywhere with the other children of Paul & Isabella. Most of them are daughters, who easily could be married by 1841 and hard to find.

I was surprised at the number of Paul FAYLEs and FELLs there are, all living at the same time in the late 18thC to early 19th. This makes it very difficult to sort them out.

Good luck with the searching and let us know if you find something.