If you're right, which is plausible and IMO more than likely, and if it turns out he was being blackmailed as opposed to the alternative possibilities you suggest, you have to wonder what it was thet they held over him to force complicity. Bearing in mind that a vast majority of those being blackmailed have been caught kiddie fiddling and the fact that Abedin felt she was closely enough linked to Graham to approach JUST HIM amidst a room full of people AND greet him warmly like an old friend or close colleague, they have to have crossed each others paths more than once previously. "Under what circumstances" is the mother of all questions in this instance.