Apologies for the late reply, was at work:
Not "hard-left", indeed may not even be Labour given the timeline is full of retweeted Lib Dem stuff (with admittedly one or two Corbyn retweets as well). Actually does live in the constituency, though.
Paul Unwin - not hard left, seems anti-Corbyn judging by the timeline full of "Corbyn GO" stuff and retweets of Dan Hodges
Robin Levett - politics aren't clear, but not hard left
One or two retweets of Momentum stuff, rather more tweets in support of Corbyn during the leadership campaign.
Lives in Aylesbury and appears to be a Lib Dem. Not hard-left, judging by the timeline.
So from your quickly-found examples, all of which date from after she was re-selected (and after your original post, fwiw), only one of them comes from someone who has previously re-tweeted Momentum stuff, and there isn't anything to say he is part of Momentum or lives / is active in the constituency.
This therefore seems to boil down therefore to a "guy in Clapham who campaigns for Labour", which perhaps may explain things given that (a) Lambeth Labour is dominated by the Progress faction and (b) its them that have tried getting rid of Kate Hoey in the past, and may well be right now given that Mandelson is actively trying to unseat her.
Do you think its fair that a party within a party like Progress should be trying to take out a popular and hardworking MP like Hoey?
I find it extremely weird that members of a party would actively seeking to help one of their incumbent MPs lose. That to me is symptomatic of the narrow mindedness of elements of the party. Labour used to be more open to different view points. At least that's how it seemed to me.