That's exactly what I'd do. I'd keep a very close eye online and in the media for exactly what people were registering as inconsistencies, likely untruths etc (and if I were the Walkers, I would absolutely, at some point, though maybe not just yet, be reading these threads and cut and pasting stuff into a Word document for notes) and crafting another book, possibly called The Real Salt Path. If my publisher were on board with that, obviously.
This would not in any way be the whole truth and nothing but the truth so help me God etc, but it would set out to be a relatable and broadly apologetic account of the circumstances that led to the omissions and half-truths of TSP. Which I would expect to be along the lines of 'We were financially irresponsible and overstretched ourselves, I did something unforgivable and stole money from someone who trusted me, and then everything snowballed. When I wrote TSP, I massaged the timeline of Moth's diagnosis and made us sound more destitute and friendless than we were because it was a better story, and I almost came to believe it myself. Lying so much was a horrible strain when the book did so well, and I always knew it was going to come out someday, particularly once the film option was actually actioned. When I see myself sitting next to Gillian Anderson on TV, I look like a deer in headlights, because we already knew The Observer had a story...'
If Michael Joseph at PRH decides to keep her on for one last book, then in her shoes, I'd couch the whole 'semi-apology' in another walk.