Corbyn seems to be determined to stubbornly ignore the antisemitism in Labour, so he will lose more MP's to the Umunna party. Labour is badly fractured, as you say, for this reason, and between Momentum and the Blairites. Jess Phillips was interviewed today and said that despite the fact that she was a Lefty, Corbyn never spoke to her because she was not part of his In-Crowd. I can really see Corbyn's Labour becoming a niche party on the Far Left.
May and the Tories success will depend on what Brexit they deliver. A good deal or transition to WTO (properly managed in liaison with the EU to ensure minimum damage to both sides) would keep the Party together (less a few more like Lee and Grieve who may go over to Umunna). But if she sells out Brexit, she will lose the ERG who may align with Farage, and I agree with you, Sooper, that Farage would have a huge following of Brexiteers claiming injustice.