Oh, that's what you disagree with. I was lost, thanks for the clarification.
The wreckage of the Queen's Gambit would constitute proof of murder. Forensics is not as flawless as TV dramatizations would have it, but they could detect that much.
Since more than one person has a book, it is likely that every member has a book and they, whoever they are, know it. Also, there still isn't any need to interrogate Oliver on the island, where he can do no harm. Dead men tell no tales. Also, there is no realistic prospect that Oliver can be treated as a hostage. Whatever the island's functions are in the grand scheme, just being there gives Oliver too much information about the existence of private armies and island prisons, etc.
Previous scenes between Moira and Merlin have indicated her agreement. I really don't think there is much ambiguity. Also, this episode heavily emphasizes the Diggle's inability to truly see his old CO. And Thea explicitly reproaches Oliver for his inability to truly see Moira. I think Oliver's failure to consider whether Moira ever knew anything, even if only told by Robert, is meant as a dramatic irony. This episode may be fairly said to be aimed at emphasizing this I think.
As to the greater horror on sexual peccadilloes, it's the infidelity accusation that gets Oliver to interrogate his mother for the first time. The woman has misplaced not one but two husbands, and that
is what he wants to talk about? Hmmm.