Thank you for this enlightening post! Being autistic myself, I have some thoughts on this as well as I see many hallmarks of autism in Viktor due to which I regard this a valid interpretation (not canon, but more on this at the end of this post). I am careful when voicing this kind of analysis as some fans get easily upset when their favourite character is interpreted in such a way. So apologies in advance in case I step on someone's toes with my findings.
Please view my post as additional data input on the subject.
First (and this is mostly directed at the NTs reading this), when talking about empathy, please note that there are several types of empathy. In the following, I’ll talk about emotional empathy and cognitive empathy. Emotional empathy works well for autistics unless we struggle with alexithymia, whereas cognitive empathy (i.e. putting oneself in someone else’s shoes) tends to be super hard. When talking about empathy regarding Viktor, he shows good emotional empathy and is good at picking up other people’s moods, i.e. when he first spots Yuuri at the banquet, when he worries about Yuuri and sensing when he’s upset, but when trying to help, he messes up because he very obviously cannot put himself in Yuuri’s shoes. However, throughout the show, he slowly becomes better at understanding Yuuri.
However, it’s also obvious that he is blissfully unaware of these issues. For if he knew about them, he would be more considerate from the start and might even communicate his differences to avoid misunderstandings. And this is where I believe that Yakov is mistaken. His harsh reaction when seen in this context, is the reaction of someone who got used to Viktor’s idiosyncrasies over the years but interprets them in a hurtful way because he doesn’t know why Viktor is the way he is. And because Viktor doesn’t know either, this is a constant source of misunderstandings and, hence, conflict. Been there, done that. It doesn’t matter how close you are. This phenomenon is called double-empathy problem and it’s a root cause of misunderstandings in autistic/allistic interactions and matches my experience pre-discovery. (I personally prefer this explanation for Yakov’s reaction, but I think both can coexist within the scope of the show).
Some more reasons why Viktor being autistic is a valid interpretation:
To me, the most prominent hallmark is actually his figure skating talent/genius and the perfectionism he exhibits in this department. (Note that this only applies to a subset of autistics, but the intensity with which we pursue our passions can lead to ingenious results.) Viktor is a once-in-a-lifetime figure skating legend. To bring his vision to the ice, he commissions music and costumes according to his parameters and choreographs his own high-difficulty programmes, which are masterpieces full of transition elements and attention to detail. It is super rare for figure skaters to do all this completely on their own, not even Yuzuru Hanyu does all this himself and I consider him a one-in-a-million talent. In addition, Viktor’s skating is super precise. All these things demand dedication. It’s hinted in the show that the major part of Viktor’s life consisted of skating and it’s not a stretch that due to this high level of dedication, eventually, he burned out and lost his inspiration over it.
Viktor is blunt to the point of being outright rude. His bluntness is also reflected in his flirting. No wonder, Yuuri runs for the hills.
Suffering from loneliness due to a lack of meaningful connection is common in autistics as we often form bonds through shared passions, which often isn’t possible with allistics to the extent we crave this. Viktor is canonically lonely (and I doubt training is the only reason for this as many top skaters practise a lot but still maintain friendships) and forms a bond with Yuuri over their shared passion. Loneliness and burnout can also cause depression (especially in autistics) and there are hints that Viktor is depressed at the beginning of the show.
Masking: Viktor masks throughout the first four episodes of the show and then starts taking off the mask because of Yuuri (the beach scene). He takes on carefully crafted personas when being around people, and doesn’t know how to be himself.
His canonical airheadedness could be well explained in this context as hyperfixating a lot on his passion or on other things crossing his mind, and basically being in his own space-time.
Making sudden life-changing decisions due to the onset of a new passion: Autistics struggle with change and there are a few hints that Viktor can’t handle it very well, too, but this trait can be overthrown by a new passion absorbing our entire existence. I see this reflected in Viktor’s decision to move to Japan and become Yuuri’s coach, which the video of Yuuri skating his free programme incites. At this point, Viktor’s long-time passion for skating has withered away, but he hasn’t yet a new passion to focus on. This kind of limbo can cause mental health problems for autistics and they can end all of a sudden when we fall in love with a new passion. Been there, done that. It’s canon that the video struck Viktor like a bolt of lightning. His passion for skating isn’t dead, but changes to the desire of being Yuuri’s coach (romantic feelings certainly play a role here as well) which ultimately leads to the reignition of his passion, or rather, a reinvention as he becomes Yuuri’s coach AND rival.
Given all this, Viktor being autistic is a valid interpretation. I’m not gonna say he IS autistic as this would mean it’s canon but it’s neither mentioned in the show nor stated by the creators. Please note that many autistic-coded characters accidentally come across as autistic because they’re either based on common autistic stereotypes with manifest as e.g. a socially awkward nerd or because their creators are undiagnosed autistics.
However, I regard this interpretation as way likelier and better backed up by the source material than interpretations of Viktor being evil/selfish/having ulterior motives towards Yuuri which I still see circulating in the fandom.
PS: Yuuri, too, exhibits several autistic traits, and some of them overlap with Viktor’s while others manifest differently. It’s a spectrum after all. And if I were to interpret both characters this way, it’s hard to unsee why they fit together so well.
Bonus funfact: Queer people are likelier to be autistic than straight people.