Disagree with your post about Riz on two fronts. First, he's no more canon ace than he is canon aro. The word of god statement from someone who doesn't even control the character isn't a definitive statement of canon. That's not how fiction works. If it's not in the text, they could justifiably walk it back at any time. Second, if anything, it's more accurate to call Riz canon aro because of his coding. A character doesn't have to say "I'm gay" to be canon gay. All they have to do is explicitly express interest exclusively in people of the same gender expression to be acknowledged as gay. Why does an aro character have to explicitly state their identity in order for it to count when that's not an expectation for allo queer people anymore? And has never been an expectation for straight people?
I agree no one should be harassing anyone, though. I'm with you there
I assumed that he talked to Murph before saying anything, but I'd hope that if someone just lied about his character, Murph would clarify. But we have no evidence either way. I would still say that someone on the team confirming it is more canon than anything implied, tho. And yep, he shows the signs, but that doesn't make it canon. He does not have a label in canon, we can say how he thinks and feels about relationships and how he reacts to those things as facts, but we cannot factually state one way or the other what his sexuality is (in the romantic sense). Anything implied is up to interpretation, which makes it unreliable. Subtext is fun, but it's like two people debating whether the color red in a painting represents love or rage, no one really wins. It's simply two different interpretations, neither is right or wrong until the artist confirms one way or another.
Something isn't canon unless it's clarified onscreen or stated to be canon by someone behind its creation. Anything else is up to interpretation.
And I think if a character explicitly shows interest in the same gender but never says they aren't attracted the other, they could be gay or bisexual. Maybe someone would get mad if you called them bi, but I wouldn't because they're just two different interpretations of the same thing.
Headcanons are valid, as long as you remember that no one is forced to have the same interpretation of media as you do. That's the great thing about media analysis. You can't be right or wrong because it's all just your interpretation.
So, in the end, Riz is not canonically Aromantic because coding doesn't equal representation and because what you see as coding could be read differently by someone with different life experiences from you.