Anything’s possible, but, to me, that’s a very odd story to make up, if Steven was not with her. Why not just say they went for a walk, he said he was going off to meet a pal , she went home and he never came back home.
With one exception, every comment I have read, here and elsewhere, and everyone I have spoken with, all say the same thing.....they find it incomprehensible that a mother would just happily set off back home without making even the most basic check to see if her son was ok and had not had some kind of accident inside the toilet.
my conclusion is that her story is true, up to a point. I just feel that what happened after 3pm is where the mystery lies.