Content warning: mention of infanticide
I have a serious issue with your description of the first one, specifically the "offensive username".
"Autistic" is not a bad word. It is not derogatory, it is not an insult, it is not offensive. Are there people who use it as such? Yes, but #!$@ them.
Is it that they might be self-identifying as autistic, and are unafraid to claim that label? If you call "self-identifying as autistic" offensive, that's really, really not okay.
Is it that they might be headcanoning Duo-the-character (possibly Duo Maxwell, possibly someone else, that's really not the point) as autistic? That, too, is not offensive, and it's really not okay to say otherwise, especially if the person originating the headcanon is autistic themself.
Do you have to agree with their headcanon? No. Is it hurting anyone or anything? No. Is it possible that an autistic person might have a much more nuanced understanding of what autism actually is, and how it looks, than an allistic (non-autistic) person? Very much so. Has a creator explicitly said that some character is allistic? Death of the Author, baby.
*sighs* Look. As you may be able to tell, this is a major hot-button for me. Hi. I'm autistic, and I find the characterization of "TheAutisticDuo" as an offensive username to itself be incredibly offensive. I don't fit the expected societal norms for an autistic person, because standard autistic stereotypes really, really aren't one-size-fits-all. There's also a lot of institutionalized sexism in autism diagnosis, which I won't get into in this post, but am happy to talk about if anyone's curious.
Organizations like Autism $peaks (which is spelled with an S, not a $, but I point-blank refuse to let the Board auto-harvest that for advertising links) are also incredibly damaging to the autistic community. I honestly wouldn't be surprised if this is where the "offensive" description ultimately originates from. They have no autistic people involved anywhere in their organization, and are primarily composed of Autism Parents TM (which is not the same as parents of autistic folks, to be clear).
Far more damning, though, is that they do stuff like supporting parents who murder their autistic children. Unfortunately, they're also the main and only """""autistic voice""""" most people are aware of. They also promote therapies like ABA, which is focused not on helping autistic folks, but rather on forcing them to pretend like they're allistic, in incredibly damaging ways. There's something like an 80% PTSD rate among ABA survivors. (I don't have sources immediately to hand, but if anyone is interested, I can dig some up. I can also go into so much more detail about Them, but I won't unless someone explicitly wants me to.)
To be clear, now that I'm past the initial "you said what" rage, I'm not actually angry at you. I'm angry at and about the rampant societal ableism and stigma surrounding autism and autistic people; I'm furious at organizations like Autism $peaks, who essentially consider my life less valuable for being autistic; and I'm heartbroken by the harm and discrimination my community faces every day.
April is Autism Awareness Month. This year, don't "light it up blue" with Autism $peaks; listen to the #ActuallyAutistic community, and go #RedInstead.
... This, I suppose, has been your Autism Awareness Month lesson. It's something I think is really important to put out there, but I'm sorry I've ended up shoving it down your throat.
~Fire Sidoni