It does get confusing. My initial covid-19 thread was closed at 1.5 replies ( 100 pages ), while the Ask a Question thread was at 2.0 replies and the favorite band thread was at 1.7 replies. The Eternal Darkness. . . thread is currently at 1.5 replies and still going strong.
The former two have been closed and restarted, but not until after questioning the difference. I do know as a former moderator that a capacity notification is received in regards to a thread's size, which is why popular ones are often closed and restarted. Therefore, it's not a matter of moderators picking random threads to lock. However, it is a moderator's discretion over when to close it once the capacity notification has been received. And, conversely, when not to.
The initial NaPo 2020 thread was notified it would be closed at 1.5 replies, yet was given a grace period to 1.8 replies ( much appreciated ) . But, then it was closed without consideration of notification, nor the opportunity to create a new one and redirect subscribers from the old to the new. This was critical, as we were still in the process of featuring Napo entrants' poetry, and had to resort to messaging until we could get it unlocked to redirect subscribers and participants ( thank you, MadameLavender ).
Knowing the whatnots of limits regarding closures would help us understand, i.e. - threads will be closed with the opportunity to restart ( at 1.5, 1.7, 2.0? replies/pages, etc. ). This simple information would explain the process so every thread starter is on equal grounds and knows what to expect ( and when ), as well as prevent biased actions against certain members by certain moderators who close member threads while leaving their own open.
As to the rest of your points, certain mindsets are always going to insert themselves into popular threads to get noticed.
Edit: I want to add that by decree of Webmiss, members are allowed to criticise the site respectfully. However, this isn't so much a criticism as request for clarification to dispel confusion.