That said, it requires some nuance and subtext awareness to ensure it comes across as authentically asking, rather than coming across a know-it-all with delusions of being Socrates, or a trial lawyer in a courtroom drama.
And if you're a recognized expert on some factual topic and are correcting some incorrect statement of fact, just assert. However very few people meet that bar, and often argue vehemently for something that later turns out to be embarrassingly wrong. Better to have just started with questions instead, especially if the topic has any degree of complexity to it.
I only meant it as a caution: it's all too easy to take one point out of a message ("ask questions instead of making assertions") and neglect the many other wonderful points you mentioned about being calm and respectful and all that.
Just kidding! Please forgive me, my friend. I only wanted to illustrate in a hopefully humorous way that just changing an assertion to a question isn't an guaranteed fix.
You did mention some key points: Don't be argumentative or threatening. Be gentle, kind and respectful. Look for ways to make your communication effective instead of triggering defensive reactions. Respect others' opinions and recognize that you may not change their views.
I would add that when someone's comment triggers something in you (I mean any of us, not you specifically), that's a good time to step back, get away from the keyboard, and go do something completely different. And really do something different that occupies your mind in a different way - don't be fuming over the online conversation the whole time.
There is a chance that when you do get back to the conversation you will have a new perspective or will at least have calmed down. You may even find that you simply don't need to reply at all.
These are all matters of the heart, and even if asking questions instead of making assertions won't automatically fix everything, it may be one good place to start. As long as you don't ask questions like the ones in my first paragraph!
So I am sorry you got downvoted for your valuable comment. I hope some of the downvoters will reverse their votes.