I don't think that Corbyn loses on the Brexit issue - Stoke is case in point: Labour won in a very pro Brexit area with a pretty poor candidate, beating both the Tories and UKIP. The fact that Corbyn supports Brexit, despite the flak he gets for it, has saved Labours bacon. The loss in Copeland for me is down to a range of factors, most notably that it was a marginal, not a safe seat as the media has portrayed it.
However, granted Labour still need to be winning these marginal seats, and I fully appreciate your angst @JakeyBoy, but I still very much believe, as naïve as it might be, that Corbyn has a better chance that anyone else, if only he could better sell his core message, which is a very attractive one - everyone wants things like protection of NHS, protection of education from the new funding formula, nationalisation of energy and rail, and so on. On this stuff he can definitely get cut through, but for me it's the failure of the media to properly report on this stuff that's the real hindrance. And, perhaps Seamus Milne being hopelessly out of depth in his role as media man. The centre ground has shifted left on political things, but to the right on cultural things so its a really hard centre ground to capture. Corbyn can capture part of it; May another, but a Blairite captures precisely none of it. A centrist politician speaks to no one right now, and wont win Labour the kind of places it needs to win.
I personally would like to see Labour split. It would be a disaster in the short term, but long term I think it would be an advantage. I reckon it takes the Tories about 10-15 years to fully kill of the NHS and maintained schools, so I would aim to be electable by 2025, when the mood is even more radical. We just have to right off 2020 I think. Labour can't do it no matter who it is.