1) What if Stein hadn't run(obviously, we'd likely have won-I agree with you on that and spent much of the post-convention period, when I wasn't out doorbelling/canvassing or at party HQ phonebanking for our ticket on social media pleading with people not to vote Stein)?
There's no disagreement between you and I about the negative effects of Stein's candidacy.

2) What if Bernie had been barred from the primaries? The results would only have been negative. It wasn't Bernie's fault that HRC's candidacy didn't connect with enough voters. You appear to think it would have changed the outcome if only HRC had started running attack ads about Trump earlier in the game, or had run further to the right. I'm not sure why you think that when no anti-Trump attack ad ever worked for any of Trump's opponents in the GOP primaries and when none of those ads in the general election campaign every produced an increase in HRC's support in any polls in the fall.

Also, the polls taken before Bernie entered the race showed HRC with 49% support nationally. She ended up winning 49%. Therefore, Bernie didn't cost HRC any votes. It's not as if there was some additional large bloc of voters somewhere who'd have voted for her if only her platform was 15 degrees further to the right and her message from the start was "I'm not a liberal". There simply weren't any swing voters between her and Trump on the issues that broke for Trump.

Why focus so much on vilifying people for '16 rather than focus on finding a positive way to win more votes for the party in '18 and '20? We can't change the past and making people admit things shouldn't have happen can't undo the past.

Finally, if, as you seem to argue, we can't afford to have real debate and real contests for Democratic nominations, how do you account for the fact that the last time we took the White House from the GOP, we did so after a presidential nomination contest that wasn't settled until a month before the convention?

We can't win by telling everyone to shut up and reducing our message to "Stop Trump!" We need to run FOR, not against. And given that most of the country is against everything Trump's doing and for something more progressive and positive instead of what he's doing, we have nothing to fear from TRYING to win the argument.