For a mother to be as depressed as you are supposing, then I would imagine there is far more at play than 'just' depression over b/feeding, IME. I certainly feel that the difficulties I had with b/feeding for the first 12 weeks with DS contributed towards the PND that I was most likely already developing/had developed. Although, the same with Hunker, I would have felt far worse had I not been able to continue, or if I had given in to the constant advice banded around to change to formula feeding. It become such a huge focus for me (which perhaps it wouldnt have, had I not been developing PND). You see, depression is pretty complicated, isnt it
As other posters have already said - PND and b/feeding difficulties, whilst one can affect the other, are two separate issues.
It could have been the case that had the PND been caught VERY early on and treated, I may have been less fixated on the b/feeding and been 'happy' to switch to formula feeding. Who knows.....? Or, maybe it wouldnt have made any difference to how I fed DS.
The facts in my case were that with much support from HV's the BFN, and a GP that recognised the correct treatment for thrush in the breast, the b/feeding finally worked out.
However, the PND continued to worsen, and wasnt diagnosed for a further 5 months. Once I started on AD's I began to feel much better.
Both can be dealt with separately, and both could have positive outcomes.
The obvious and straightforward answer that I suspect you are looking for isnt really there. Simply because there isnt a straightforward cause to depression.
However, there often is with b/feeding issues.
That said, both are often misdiagnosed, or missed altogether.
So, each situation on its own merit, I think.