Until this seemingly mythical party and leader comes along, the answer to the first question is almost anything you want it to be. I take your lesson learning request on board and agree to an extent. Personally, I've learned that the Labour party only speaks for me with someone like Corbyn in charge. The pre-Corbyn nodding along with austerity, tougher than the Tories on welfare, not representing those out of work party was as close to Red Tory as it gets and why I use the term to describe anyone who looks back at those days while Gladys Knight sings The Way We Were in their head. If you can look at that, then add the PLP's actions over the last 2 years and all the journalists they run to when they have a story to tell and convince me that the party wouldn't immediately go back there the second Corbyn goes, I hope you're in the selling business because you'd genuinely have missed your calling in life if you're not. Heck, I'd probably vote for you as a politician.
I would question why you want to restrict lesson learning to Corbyn's time though and not even as far back as Blair and the millions of votes he shed even while winning and then through Brown, the apparently more electable Miliband brother and even Scottish Labour. Doing that is what led me to abandoning the party until Corbyn appeared and offered something different. But you can bet that the people who are responsible for all of Labour's failures before and during that leadership election would be hanging around, either in the party, as a MP or getting their face and opinion into the press, whoever sits in the leadership position the day after the election.