This whole thing baffles me. First off, I assume Ms Rayner will be lodging a claim with her specialist tax adviser's liability insurance for giving factually inaccurate advice on the rules, which are clearly laid out in publicly-available HMRC manuals.
Second, I remain deeply curious as to how she got a £640k mortgage in the first place, even with a £160k deposit. She may be a minister but she's not paid THAT well (at least in terms of official salary for mortgage purposes, I have no idea of what she makes on the side)