Interesting.
If Corbyn did start a new party would that kick off a new round of Corbyn demonisation in the media or would they stay quiet to facilitate a split Labour vote?
New parties never get very far; the last one to do so was the Labour party 100 years ago and they did it by moving into ground previously held by the Liberal Party. Any attempts since then have been failures.
The Red Wall constituencies comprehensively rejected Corbynism in 2019 and although polling shows that they are moving back to Labour I don't think they'd welcome Corbyn back. I'm not sure about the London constituencies. I think the the LibDems might be the winners there if the Labour vote is split.
The tory party is interesting, too. They are very split, with the small state, deregulation, low tax advocates in the ascendancy by virtue of the fact that there are sufficient of them to destroy the tory majority. Johnson is not the man to hold the party together; he has no principles, no real authority (he needed Labour support to get his mild set of covid restrictions through parliament) and no leadership qualities. How would the electorate react to the policies being pursued by the large tail that is wagging him if he gave in to them to retain power ? And, if he gives into them will the more reasonable tories go along with the 'remain in power at any cost' doctrine?