I am currently re-watching Season 4 and have got to the start of the Riley Jenkins episodes.
I am struck by Diana’s repeated assertion of “ You were in danger. A mother knows” and how it could relate to William Reid potentially sexually abusing Spencer, which has been discussed here in quite a lot of detail. He comes in, sits on Spencer’s bed and Spencer pretends to be asleep, plus some other details I cannot recall make it seem “off” , coupled with Spencer having completely blocked out his childhood and perhaps now, subconsciously pinning the blame on the male he is genetically related to for Riley Jenkins is his way of dealing with it. The fact that William just leaves and that Diana, in her lucid moments, never thinks to have Spencer spend time with him just makes me suspect that perhaps she thought William could be molesting Spencer.