Yeah like I say, the parachuting in of candidates by the leadership is annoying regardless who is doing it. Our previous MP was parachuted in as as an ideological ally of Blair, our current MP was parachuted in as an ideological ally of Corbyn. I've never had the opportunity to vote for a Labour candidate who actually lived here before they were elected. Mandatory reselection would have changed that as CLPs would have had the power to replace candidates who they don't feel represent them.
It also would have applied equally to everyone. Under mandatory reselection Corbyn could have been deselected if that's what members of his CLP wanted. The right feared it because, as I said in my previous post, they knew full well that they aren't good at inspiring people to join the party and they aren't good at winning hearts and minds. The only way the right can maintain control of the party is by making the party as undemocratic as possible (e.g - Blair parachuting dozens of ideological allies into safe seats/neutering the party conference or Starmer now trying to change the rules so the members have less say in who leads the party) or by lying through their teeth to get elected (e.g - Watson, Starmer).
You raise Watson as someone subjected to nastiness by the left. The left certainly wasn't being nasty to Watson when they elected him as Deputy Leader in 2015. The left's dislike of Watson stems from when, immediately after they elected him, he called them thugs and agitated for them to be booted from the party. Again, this is the problem with both sides-ing the issue. The right attacks, undermines and smears the left and no-one gives a shit whilst the left is blamed for standing up for itself and held collectively responsible for the actions of an unpleasant minority. This is also why your take on the current situation is so skewed, to your mind complaining on Twitter about a guy getting elected on a unity ticket and embarking on a massive factional assault is just as bad as being the guy who got elected on a unity ticket and embarked on a massive factional assault. It's just rank double standards. When a left wing leader who actively tries to foster party unity comes in, the right can spend 5 years actively working to trash the party's electoral chances with impunity, but when a centrist leader comes in and actively attacks the left, the left are expected to stay quiet, keep paying their membership dues and come out to volunteer at election time.
I'm not going to get into a few of your other points as I've written long posts about my views on the outcomes of the 2017 and 2019 elections before (which incidentally contain a lot of critique of the Corbyn leadership) but on the purges/targeting of the left,
@ZupZup has given a recent high-profile example. Another recent one would be Kate Osbourne, a socialist Labour MP who recently received a letter informing her she was under investigation which was later rescinded as there were no grounds to investigate her. There are thousands of these cases spanning years and in very few cases do the victims have the ability to challenge them in the way higher profile people can, one guy during the 2015 leadership election was barred from voting for an abusive tweet which turned out to read "I fecking love the Foo Fighters".
