The level of proof required in these courts is a lot lower than criminal cases: it's on balance of probability. Corbyn's brief could show plenty of examples where a sitting MP has left their party and stood either as an independent or for another party at the next election and lost: the number of MPs who win in such circumstances is a very short list. See Chukka Ummuna (sp?) and all the other retards who were party-hopping over the last few years: all lost, despite some of them previously having huge majorities.
Don't forget that Corbyn is also anxious because of the Boundary Commission. If - as seems possible - Boris doesn't wait until the 2023 review and opts to bring in the 2018 changes proposed by the Commission, Islington North ceases to exist as a constituency.
Including his, there are five Labour-held seats in London which disappear - so he was already in the position of having to hope that either a local constituency party which had effectively succumbed to Momentum entryism and which is a safe seat would deselect its MP and invite him in rather than holding a selection contest - which he'd probably win, in truth - (and that this would be permitted by Starmer) or a constituency where the MP was planning to retire selected him.
He'd otherwise have been left to hope that Diane Abbott would retire and the local party - already Corbynite and bolstered by more Corbynites - would select him. (Abbott's constituency also goes, but the wards which make up much of her current seat will be part of the new one, along with wards from St Jeremy's current constituency, making it safe Labour).
Not being a sitting Labour MP makes his task harder - incumbent MPs are almost always given the chance to run for a new seat, and one which they might at least stand a chance of winning - but Starmer is unlikely to feel the slightest compunction to observe this nicety for an Independent MP. I doubt that he'd be too keen on offering JC this bonus were he still taking the Labour whip, but, without it...
...Corbyn can see his nice little seat in the establishment (he's the sixth most senior MP) disappearing, and since the most vociferous anti-establishment candidates rarely willingly retire from the establishment they spend all their time criticising this matters not just on principle, but because he's highly unlikely to want to spend more time with his allotment. Not winning this means that he's either going to have to apologise in full for his behaviour and hope that Starmer restores the whip, or to form the Popular Front for the Liberation of Tufnell Park & Palestine, and run, Galloway-like, in a seat held by a Labour MP who's not particularly popular with the locals and where the Tories and Lib Dems will never get a look in.
Mentioning Galloway reminds me that his argument that he's effectively being prevented from becoming an MP as an independent runs into bother - as well as Galloway winning twice when was leading a party which was little more than him and a couple of people who make Lenin look like a High Tory, there's also the case of Peter Law at Blaenau Gwent. Llew Smith decided to retire, and in a bid to get a more pliant MP, Blair imposed Maggie Jones on the constituency via an all-female shortlist. Law was the Welsh Assembly member for the constituency and after making clear that the shortlist was cover for a stitch up (all the women just happened to be Blairites), he left the party and ran as an independent, overturning a 19,000 Labour majority. When he died less than 12 months later, Owen Smith - remember him? - was chosen as the Labour candidate, but lost to Law's election agent as the constituency hadn't finished punishing Blair...
Both cases mean that Corbynites' constant trumpeting of his greatness and how many supporters he has will also count against him, since Labour can demonstrate that someone with much popular support drawn from the hard-of-thinking, the credulous, the naive and the downright dangerous can win a seat (for clarity, Galloway, not Law who seemed a reasonable sort), so why does he need the cachet of the Labour whip...?