I pretty much agree with the whole thing being said about minority characters.
The only thing I have to add, and it's not necessarily related to minority characters, is that yeah, speculation/headcanons can be a bad thing when done wrong. I've seen it happen plenty of times, where "headcanons" become too accepted when they aren't actually true, and the general perception of the character becomes warped as a result.
As I've said before, a headcanon is not a headcanon if it isn't well backed up by official material, and if it's been flat out said it can't be the case by an official source (like the creator of the character). "Samus is trans" isn't and cannot be a headcanon because nothing about her character suggests it, and it's already been flat out said that it can't be the case by someone deeply involved with the character's portrayal. If you seriously believe Samus is trans and want to stick with that regardless of official material, then you have a fanon - essentially a made up version of what the character is.
People seem to not understand this distinction and get upset when you tell them they can't headcanon something, thinking it's mean, but it's not. You can't just make up anything you want about a character and call it a headcanon. That's just how it is. If people understood this, far less characters would be misunderstood and misrepresented in fan-art and what not.