I would suggest that the couple of hundred people in Westminster have their feet on the ground, and realise no matter how much the ultra-left support Corbyn, the vast majority of the population do not.
At the moment, I should imagine that the vast majority of the PLP are more anxious about their individual seats, than they are about a Labour election victory. Corbyn's Shadow Cabinet appointments have made it clear that he is extraordinarily vindictive, sane Labour MPs will be very worried about deselection by the ultra-left.
The lunatics have taken over the asylum in the Labour Party, and no one seems to have the inclination, or energy to remove them. It is Militant all over again, except that this time they have their man in the driving seat.
You are absolutely right though, when you say that this continued fuckwittery on the part of Labour, is giving the Conservatives a free run.