I don't think they made up symptoms in 2015. Reading the consultant's letter, he describes subtle signs that you just couldn't fake (eg saccadic eye movements).
I think they may have gone for a long walk in 2013-14. SWs mother died in early 2015, then TW got this diagnosis of possible mild, indolent disease later in 2015.
Following these two events, SW probably got writing, and juggled timelines around and exaggerated the gravity of the situation to give extra interest to the narrative of running from the bailiffs, sorry, I mean being homeless, in 2013. (And her editors might not have discouraged this).
That's my reading of the situation.