I can't imagine Putin allows for much dissent among his underlings when he assassinates reporters that disagree with him.
Second point makes sense explaining the overall goal for Russian involvement in America, but it ultimately requires that the video (or anything similarly damning) actually exists for it to matter. Also doesn't really explain the specific reason for releasing info to this one ex-MI6 guy. Unless the memo itself is just intended to cause controversy and polarize American politics further, but if that's the case it doesn't even matter if reports are factual, because the damage is still being done.