The issue with this I think is that even if Cannan was posing as a businessman and trying to do some sort of deal with her - be it legit or scam - it doesn’t explain why it ends in murder.
If she’s backing out then how much of an inconvenience is that for Cannan? He’s not got a business empire to lose, nor a marriage that might fail if the truth comes out - there’s no need to silence her, which would surely be the motive here.
Indeed, she might threaten to go to the police, but victims of scams are often reluctant to speak up, “how could you be so stupid Susannah, the man’s a crook?!” etc etc. Her reputation matters far more than his, she’s the yuppie, popular member of the Putney set - even tangentially being linked to anything dodgy will likely reflect negatively on her. Whereas, he’s an outsider who can scarper at a moment’s notice.
And that’s if she can even make anything stick - what ‘evidence’ in terms of a paper trail does she have? Does she even have contact details for this guy? A name (one that’s not fake, anyway)? Anything at all that can be traced? Seems unlikely, given that whatever was in her diary didn’t lead to him.
Killing her in a fit of rage is more believable, yet the theory seems to go that he secured himself somewhere quiet in order to kill her, to conceal then dispose of her body, possibly with the support of an accomplice. That suggests premeditation, rather than a spur of the moment attack.
I do think the deal angle is a promising one but I think it works better if tied to someone with something to lose, who had motive for wanting her dead.