So I think it's about time I add in my two cents...
I have a feeling you are referencing some of these cases on this very forum, and they are definitely good examples of how you can conduct yourself with the moderation of your own community.
The larger a community gets, the more that these cases may come up and it definitely increases the risk of poor behaviour and conduct by some of your users. At the same time it can also increase the negativity from other users that don't necessarily believe in the same thing and/or try to push their own opinions to (try to) influence others. The main thing to have is a central and general standard that ALL of your users should be going by. This can be done with forum rules or guidelines like we have here. There are some communities that have them on a page as we do, but others have it as part of their registration process where you need to agree to following the guidelines/rules before you can actually create your account. Not everyone will read them and that's normal; I deal with finance contracts on a daily basis in my employment with my customers and they don't even read that! So expect people to simply skim over them or not read them at all, but definitely enforce them in a manner that comes across in a positive or constructive manner as opposed to "READ THE RULES OR BE BANNED!" if you know what I mean...
Which brings me to my next point, patience or overall attitude.
You must treat every single user the exact same. You cannot promote favouritism or allow one person to get away with it and pin the next person that makes the same mistake. If you do allow that, you will give people the impression that they can get away with it and give them the green flag to point fingers to other users that were not pulled up on their actions elsewhere in your community. Not only will this make things go pear-shaped quick smart, but you will eventually develop a toxic community, and that is something that is a lot harder to deal with than the initial issue. You are also dealing with users of different ages, backgrounds, life experiences, expertise, etc, so giving yourself the head start in making sure to keep a level head will always come across better than simply having your administrator/moderator hat and badge on. Be yourself and be human. Don't act like someone you're not and if you are unsure, consult your own staff team, colleagues or others that hold similar positions on other forums and communities.
To answer some of the things you have highlighted in your post... I speak from my own personal experiences and how I conduct myself on Discuss of course, so by no means do I speak on behalf of my colleagues or the senior management of Flarum. Whenever I see someone playing up or going against the guidelines, I simply post a reminder and advise them of what they're doing no matter how obvious it may be. There are some core things that I do not condone such as condescending behaviour, swearing, offensive posts, continuous hate posts made about a person, product or feature and even something as simple as someone enforcing their political and religious views. My approach to this is to give them a verbal warning (whether it be posting directly in the discussion and removing negative content or warning them privately). If things persist, then just go up the ladder based on how serious the incident is. The only time I ever blind ban someone is for spam accounts, nudity and other serious offences like that. As a user, I would hate to be banned without some sort of explanation or at least be given the opportunity to address and improve my actions. Not everyone will get the hint, but they are dealt with accordingly if they continue to play up as you would have seen here from time to time as well.
Replying to these posts wouldn't necessarily promote more negative behaviour, but sometimes with language barriers it may not come across as intended. When you meet someone in person, you are able to pick up on tone, emotion, body language, etc. On a forum, all we have is text to read, capitalisation, bold, italic, etc. It doesn't really give us the power to use these other ways you would normally communicate with people to see what angle they're coming from and how they want to come across. This would be the same with leaving posts unapproved; I would at least address the post originator of why their post wouldn't be approved and them dismiss/delete it. No reason to have it in limbo if it's not going to do anything for you.
Hope this helps you. Happy to answer any questions you may have. 😉