Whilst I've found out a bit it just continues to muddy the waters!
In 1841 the 3 Nash children - James, Ann and Isabella would appear to me to all have been born as Nash to a James and Sarah:
James Nash bp.2/7/1823 Stafford, Worcestershire son of James and Sarah
Ann Nash bp.18/10/1827 Ombersley, Worcestershire dau. of James and Sarah
Isabella Nash bp.22/11/1833 Bromsgrove, Worcestershire, dau. of James and Sarah.
Don't know the occupation of James Nash but clearly it moved the family around.
In 1841 Sarah and the children now in Old Swinford - Sarah is head of the household and living with them is James Painter 40 as you already know.
Bearing in mind the first child was born in Stafford, Worcs. this looks a likely marriage:
James Nash married Sarah Rennells 14/1/1822 Stafford, Worcs.
So what happened to James Nash senior - can't find a obvious death before 1841 but certainly he doesn't seem to be around and living with Sarah and the children is James Painter.
I can't say that the Emma Painter baptised 1837 is Isabella (Emma) being rebaptised to James and Sarah Painter. Definitely no marriage between James Painter and Sarah Nash.
You say that Isabella Emma was born in Old Swinford, but half of the census' I've found her on state Bromsgrove which ties in with the baptism of Isabella Nash.
Isabella may have had no memory of father James Nash and only remembered James Painter (who appears to have died I think in 1843). It's a sticky one, isn't it?
Two years after the death of James Painter Sarah Nash remarries:
John Round, widower married Sarah Nash, widow 2/11/1845 St. Thomas, Dudley.
According to census Sarah Round, formerly Nash, nee Rennells was born in Dudley, Worcs.
My instinct here is that Isabella was really a Nash, not Painter, although perhaps her only father figure was James Painter (she'd only have been 10 when he died).
I haven't found her in 1851 so don't know if she was Nash then, or Painter.
Can't solve the dilemma of Nash/Painter but thought I'd show my findings.
Annette