Her editor had presumably advised taking her mother's death out of TSP, as it didn't work with the 'walking towards healing' narrative and started off questions in the reader's mind about what their relationships with their families were, why no help or a roof over their heads were forthcoming etc.
I think she was right to take it out. The whole thrust of TWS is that they were essentially alone in the world.
Insert a dying mother and not only do you have questions about how SW got to her mother's deathbed from the SWCP and where was Moth while she did so, but also why staying with her mother wasn't an option when they lost their home, where were TW's parents, did either have siblings other than TW's brother who let them use his address for post etc.
Then, after the success of TSP, when it was clear there was an appetite for answers about what happened next and a sequel, SW is up against all kinds of issues, like a lack of obvious narrative and her rearranged diagnostic timeline for TW.
In reality the medical letters in her statement suggest he was first tentatively diagnosed with CBD/S in June 2015 (after the SWCP walk), but was untroubled enough by symptoms to start a (presumably hands-on/physical) horticulture diploma or degree that autumn, which doesn't fit at all with TWS's depiction of a man whose condition is worsening daily to the point where he drives for hours in the wrong direction, and needs a 'to do' list to remind him exactly what to do.
Similarly, her mother's death, which seems to have happened in reality 'after' the events of TSP and 'before' the events depicted in TWS, is incorporated into TWS as though it's happening during Moth's second term at university, I think as a dramatic device.
TW is gone all day, SW appears not to have a job or a purpose, and there's not enough content for a book. Her mother's death allows her to physically put herself back in the midlands, use the physicality of her death to catastrophise about TW's demise, but in fact her mother's death is pretty tokenistic. It's just there so that she can talk about her childhood, her attunement to the land, and how she met TW.