the 1841 census does not show birth parishes
the 1851 has Castletown for whole family - possibly it was a lazy enumerator - I can find no matching bapt in castletown/malew thus I suspect the 1861 is probably more accurate.

If you want to look at Bishop's transcripts they along with all other records are held at Manx Museum - they open 10am - 5pm Tues thru Sat - Tues is generally very busy - if you want to see any specific item I suggest email library@mnh.gov.im

many parish registers etc are now on line see www.imuseum.im

however there were 2 James Christians born in 1838 to a father Wm in Castletown - how sure are you of the link back to your painter Wm (other than James was also a painter) .

Finding the burial for Jane would confirm - I suspect she went to live in England with one of her children + died there.

If my suggestion is correct that Wm was younger son of a small farmer then he would not inherit property but his father would have done and this is the record that can be traced