I have a vague and unhelpful answer, and I imagine it's the only real answer to this question, but just floating it to get others' perspectives.

I imagine the answer is "An edit is too invasive when it violates the intent of the author." When you present them as saying something they didn't mean to say. I'd say it's too invasive even if it loses their tone (sometimes grammar/usage/typo edits can be unwise if they're not simply making understanding possible).

Which of course raises the question "Well who (aside from the original author) is to say what they intended? Isn't it a subjective call, when something crosses that line?" Yeah, it would be. I don't know that there's any better answer than "you know it when you see it."

I have a vague and unhelpful answer, and I imagine it's the only real answer to this question, but just floating it to get others' perspectives.

I imagine the answer is "An edit is too invasive when it violates the intent of the author." When you present them as saying something they didn't mean to say. I'd say it's too invasive even if it loses their tone (sometimes grammar/usage/typo edits can be unwise if they're not simply making understanding possible).

Which of course raises the question "Well who (aside from the original author) is to say what they intended? Isn't it a subjective call, when something crosses that line?" Yeah, it would be. I don't know that there's any better answer than "you know it when you see it."

Also helpful to the conversation: This 13-year-old Q/A from meta.stackexchange.com, from the formative days, pretty well sums it up:

You edit to make things better, clearer, more effective -- never to change meaning.