As leader of HM Opposition, only he can try to form the alternative government in September/October.
Does he have to be leader of the Opposition for that purpose? I doubt it. I’m sure a neutral could be leader of the Opposition for the purposes under discussion. Ken Clark, Chukka or Vince Cable. Corbyn could remain leader of the Labour party.
When all the dust had settled a GE would be called and Corbyn would campaign as normal.
It’s pretty clear that Corbyn is the sole impediment to an organized multi party takeover by the opposition. [1] If he (temporarily) stepped out of the way all objections would be overcome. [1]
Corbyn’s a brexiteer and he’s doing the best he can to ensure a ‘proper’ Brexit happens. Personally I think that’s commendable – he’s doing what he (perhaps misguidedly) thinks is right.
[1] Assuming they have the numbers which isn’t clear.