recollections may vary = we identified the the person and spoke to them. They did not mean it the way it was taken. So the RF knows who it is, so why don't they tell us and explain what this person meant actually.
Problem is there were various conversations.
The 'palace sources' have not give us the name, so it's someone protected, so forget PA and Sarah. They would be thrown under the bus in a second. Unlikely that it's Kate whom MM described as a good person. Now and again, the press puts out snarky little reports and 'polls' about bypassing Charles - presumably a warning to PC, that after waiting so long, we can bring you down. So either the 'sources' said it was PC and this is used as leverage for future stories etc. Same could be said for W.
A colleague once told me how she saw a partner of someone she knew, in a situation that they thought he was cheating. She went over to him, so he knows that she knew. She said she did this to give him the chance to explain himself and his side of the story as she knew that it would come out all wrong for her to tell the friend herself.