And that's exactly what the sportsmanship bullet point is saying. Don't be a jerk. And you proved why treating people how you want to be treated is the way to go. If, deep down, you don't want to deal with jerks, don't be a jerk yourself. Bad attitudes like that are contagious, spreading and sucking the fun from a play session. Better to set an example of sportsmanship and friendliness to get similar behavior. Yeah, you have players who are determined to be a-holes, but I've found being polite and bantering with my opponents has encouraged similar behavior from them.

Exactly so. If I realise I've missed an opportunity, I go "Oh, bugger" and carry on to the next thing. In fact, my personal preference is to try and hold myself to getting everything right, even if I let my opponent take something back.

This works both ways; using "fly casual" as some sort of battleflag and claiming that you should be allowed to undo a stupid mistake that you've just realised you made because of 'not being a tournament rules nazi' can be, in its own way, just as aggressive. The rules are the rules, and, in a competetive environment, apply to you unless your opponent choses to grant you an exception. 'Take-backs' are not an automatic right, and it is just as possible to play a fun game strictly to the rules.

Not letting someone take back a move is fine if you hold yourself to the same standard. What's not fine is letting things pass for the first couple of turns and then insisting once a critical thing occurs after having been fine with them doing so up until that point. Because that's changing the 'terms' of the game partway through.