There is a Sally Mortimer baptised 14th March 1756 at Charles the Martyr, (the same church as the marriage to Joseph Bounsall in 1789), to a Peter and Sarah Mortimer.
'Sally' can be a variation of 'Sarah'
Unfortunately, I then found a burial of a 'Sarah Mortimer', 'daughter' of Peter on 30th March 1760, so that seems to scupper that one.
I see on the actual image in the baptism register for Catherine in 1798, in whom there seems to be some dispute as to her parents, it says quite plainly she is the child of Joseph and Sarah Bounsall. I would have thought if there was any doubt about her parentage, for example if she was illegitimate, there would have been something written in the register. Most likely just the mother's name recorded, and some comment made.
Whilst searching, I found one of the witnesses on the marriage of Joseph and Sarah in 1789, was a James Bounsall.
James seems to marry in 1798 at Stoke Damerel - just a mile or so out of Plymouth itself, and Joseph signs as a witness.
I think these are the same signatures on the two marriage entries, despite James writing 'James' in one, and 'Jas' in the other - a common abbreviation for James.
They are both 'Taylors', so I suspect they are brothers. You may well have already found this of course.
This doesn't help with Sarah Mortimore/Mortimer.
She may not have been a Plymouth girl of course, and come from some other part of the county to the big city to work, or from Cornwall, just over the river in Plymouth, or even further afield.
There is a Sarah Mortimer baptised in 1778 in the Isles of Scilly, just off the tip of Cornwall, father Thomas mother Sarah, but this would make her too young to be married in 1789. Unfortunately again, there is a burial of of a Sarah Mortimer in 1779, and another baptism of a Sarah to Thomas and Sarah in 1788, so again this is not her.
Normally, I would be looking for wills for any one related to the families. They can really help sort out problems if they can be found, but as you may know, a good proportion of Devon wills were destroyed during WWII when Exeter Cathedral was bombed. You may be lucky and find some that were transcribed before the bombing, or extracts of them, or any made at the Prerogative Court of Canterbury (available on Ancestry or National Archives Discovery Site for free at the moment if you don't have a sub to Ancestry).
Do you know when the father of Joseph's first wife died? Perhaps he left bequests to his grandchildren, and included his step grandchildren. Or even other members of her family. Spinster aunts and bachelor uncles were very good at leaving bequests to nephews and nieces.
What about Joseph Bounsall himself, or his parents, or his siblings? Are they any wills for them? As well as bequests in the wills themselves, sometimes witnesses to the will or Inventory appraisers can also give clues.
Good luck!
