No, stj, I understood everything you wrote down but dont think you and I are watching the same show.
Why do you think the infidelities didnt go down well? It would be a common enough thought by a young girl traumatized by her father and brothers deaths that something was going on with her mom and the guy she is sneaking around with. Or did you not like the way it was so abruptly introduced?
The island timeline is I would say about 3 weeks into Olivers arrival there. Certainly not much longer. I also dont know if Mentor knows about the book as Oliver only discovered it had writing on it in the cave when he was trapped there. So they are keeping Oliver alive to see if his father told him anything not that he gave him a book. Probably also keeping him alive as leverage against the mother should they need it in the future.
The plan was only to kill Robert and I doubt Moira sanctioned that any more than she did having Walter kidnapped. Oliver being on the ship was probably not planned for at all, he just went aboard with the sister to do the deed away from Laurals prying eyes.
Raising the Gambit was a poor idea on Moiras part as she could not prove that its sinking was anything more than an accident. Merlin killed the security guy just as a precaution. Moira as written is a very weak character unsuited to the life of intrigue she is involved with.