1. Brexit - an error of political judgment
I was of the opinion that Labour lost the election when it prevented the passage of Theresa May's Withdrawal Act. Considered purely from the point of view of winning elections, leaving aside questions of 'right' or 'wrong', I think that might have been one of the worst decisions Labour ever made. Labour was not going to win if Remain was still on the table as a possible option. Party members were on the whole pro-Remain - this was a point on which 'New Labour' and Momentum were in agreement, and the new post-Corbyn membership (whatever about Corbyn himself) was broadly in agreement with the PLP. But the Leave vote was one of the rare occasions on which the old English and Welsh working class, demoralised after years of de-industrialisation, had been able to make its feelings known and to feel it had gained a victory. It was absolutely essential to the success of the Corbyn project to convince these people that the long years of neglect and contempt they had endured at the hands of the Labour Party had come to an end.
This created a contradiction at the heart of Labour politics that could not be resolved. But it could, to a large extent, have been evaded had May's deal gone through and Remain been taken off the table. Labour had already bitten the bullet in voting for the implementation of Article 50. The 2017 Manifesto committed us to respecting the result of the referendum. It would have been quite consistent with that commitment to have either supported the passage of the Withdrawal Act or at least to have abstained.
While opposing the act Labour never to my knowledge developed an argument against it (essentially it preserved the status quo with regard to Europe during the transition period creating a strong presumption that the final deal would be a soft brexit). So far as I could see, Labour's only quarrel was with the non-binding political declaration and not to anything that was in it (it was largely a list of things that would need to be negotiated) but because it didn't contain much in the way of firm policy commitments - probably just as well given that the government that would have made these commitments was a Tory government.
My own preference would have been that, once the referendum was lost (and I had campaigned quite vigorously on the Remain side) the party would have been able to see the possible advantages of Brexit in terms of re-establishing a national economy in accordance with the best slogan produced during Corbyn's leadership - not the dreary meaningless New Labour 'For the many not the few' but 'Rebuilding Britain'. But given the composition of both membership and PLP that may have been too much to ask.