Not debating any point here I know less than you about the internal workings I did hear on Daily Politics that momentum are moving in to take over. The leadership battle is one battle the PLP is another, two battles and the war is won.
There are three battles, the leadership, the PLP and the CLPs.
Currently Corbyn has won the leadership (whether that remains we may see today), the PLP is currently massively lost (hence the overwhelming vote of no confidence). The CLPs is perhaps the most interesting battle ground, with a view that winning the CLP battle will ultimately result in winning the PLP battle as centrist MPs are deselected and replaced by Corbynites.
Now I think that will be much harder than many imagine. First CLPs tend to be very loyal to their MP - even if they are somewhat ideologically at odds with the membership in that constituency. There is a kind of equivalent to the 'I kind stand tories, but my mate who is a tory is great'. So given that the constituency will have worked together with the MP to get them elected (perhaps several times) they tend to be rather loyal and supportive. Don't forget that the membership will likely know the MP well, have socialised with him or her, perhaps visited Parliament with them and will be, to an extent, in awe of the notion that this person is an MP. Coupled with that, most MPs tend to be be pretty hard working so they tend to command respect in their local party for their position and hard work, regardless of their ideology.
So I don't think it is at all a given that the Streatham CLP would deselect Umunna, nor Leeds Central deselect Benn, nor Stoke central deselect Hunt.
And there is another point - to deselect their sitting MP the CLP would need to select someone else - and they have to agree. Almost by definition this will be someone much less experienced that the obvious candidate (their sitting MP) and likely there would be no one person commanding universal support as an alternative to the sitting MP. I've seen this myself when a sitting MP decided not to stand again and all hell breaks lose between rival factions.
The final point is about actual electability. Again, by definition a sitting MP has been popular enough amongst the wider electorate to be voted in - and in most cases there is an incumbency bonus, in other words a sitting MP gains perhaps 1000 votes extra on the basis that they are simply the current MP, with high recognition factor, plus some level of personal vote from people they've helped who might not otherwise be natural supporters. CLPs know this very, very well so they run the risk that by deselecting their sitting MP (an election winner) that they lose that bonus, plus hack off a portion of the wider electorate who don't understand why are few extreme activists are preventing them from voting for their popular MP and putting up someone they don't know or trust instead. Result, in the more marginal constituencies - deselect your sitting MP, replace with a Corbynite and end up with a tory MP, or a UKIP etc after the election.