Regarding the name she writes under.
That must have come from her and Tim.
I very much doubt she'd be advertising herself as Sally Walker to all and sundry.
At least with a name change, most people who didn't read a book written by Raynor Winn wouldn't connect any publicity they may have come across to a Sally Walker who absconded owing money. Unless they saw her picture.
If she had written under her own name, ears would have pricked up, and a lot more people would have been aware, and potentially decided to pursue her for what she owed.
After all, in 2018 when TSP was published, a lot of her debts would have been within the 6 year cut off period.
So I don't think that came from her agent or publisher. I think it came from her and Tim having a discussion and choosing new names for themselves before she went looking to sell her writing.