AIBU to struggle with having to hide my genuine opinions about colleagues at work?

Firstly I'd suggest you get rid of the notion that being 'your authentic self' at all times is either necessary or desirable. At work it's your professional self that's required and sometimes that does mean adopting or concealing behaviour that feels 'authentic', because its not utilitarian in that environment.
Do you imagine that everyone sitting in the meeting with the boss is really enthusiastic about the project? That no one is bored to tears or who would rather it was shelved simply because they don't want to be bothered to do the work? Yet no one says so. They sit up with their listening face on, and nod along, only speaking up politely if they really have an issue.
Dealing with colleagues you don't like is exactly the same.

1. How do you work alongside people you genuinely dislike without feeling you're constantly pretending? - if you dislike them as in despise them and everything they stand for, and also have to actively collaborate with them regularly, it's incredibly difficult. If you simply are not that keen on them or don't have much in common, and are working alongside rather than with them, it's a lot easier. It comes down to being polite but minimalistic in interactions. I don't see politeness as pretending because I believe that everyone is entitled to basic politeness unless they are behaving appallingly. If you are actively faking a big interest in their interests and pretending to take pleasure in their company when you don't then you are pretending and are obviously going to feel like you're pretending.

2. Is it acceptable to be polite but emotionally distant, rather than friendly? - yes, unless you work somewhere where networking is crucial to success. As a bad faker, I would not do well in this sort of job. I would argue that we don't need most people to be our friends at work or in life generally and it's generally a waste of energy to fake interests we don't have unless we have a very specific rationale for doing so. For one thing, it's cruel as if the other person falls for it, they're getting encouraged to go on about stuff we find boring and that is sort of mean in a way.

3. How do you set boundaries with colleagues who irritate you without creating conflict? - you are polite but direct. 'I would really like it if you would do x / stop doing y.' Add a concise reason if you want to. With the work space for example I would simply state that I like to have lots of things out at once and it suits my style of working. If they violate your boundaries / ignore them, you can draw attention to what you previously requested. The trouble is that setting a boundary even politely can be read as rude by some people and might lead them to like you less (naturally people tend to like being able to walk over others or not filter themselves, and boundaries can make people feel self/-conscioys, defensive, embarrassed, irritated etc). So it all depends on whether this is worth the risk to you.

4. How do other autistic people cope with the expectation to filter their genuine opinions at work? - am not autistic so can't answer this one but I asked my autistic sibling who says that being polite is just part of getting by for everyone and he chooses jobs where interaction with others isn't continuously necessary so that he can have a break.

5. How do you distinguish between being authentic and being unnecessarily blunt? - to me authenticity is about what we think and feel and believe, and acting in a way that fits with that as much as possible. We all probably have bits of ourselves that conflict, eg the preference 'I don't like materialistic people' might motivate you to want to be rude to someone but rudeness could conflict with the value 'Don't treat people like dirt'. No one gets to be fully authentic and unfiltered in how we express ourselves if we want to get on in the world and with others. On the other hand, I do think that firm, honest directness can be mistaken for overly blunt rudeness, especially when it comes from a woman or a member of certain racialised minority groups, and that this is really unfair. Which one people think it is will depend on context. It can be especially hard to start setting boundaries when people aren't used to it because they are likely to be surprised.

You don’t have to give your genuine opinion on anything. You simply don’t have to.

I have 3 coworkers who I deeply, deeply dislike. I keep every interaction work focused, mind my own business and don’t give them any opening. That’s it. It works and I’ve never been involved in any workplace drama with them.

Totally agree. We are ' masking' our real selves much of the time.

I “listen” a lot and just make noncommittal noises as “engagement”.
Then I just keep it professional, polite and helpful.
I focus A LOT on the work and find extra stuff to do if needed . Since I work in a school, that’s pretty easy. There’s always extra stuff to do.

It works 99% of the time. When it doesn’t, it gets ignored as I’m normally so professional, polite and helpful, so I must just be having a bad day. It is exhausting though.

yup. That is what it is like for everybody.
I would also say that working with people you dislike is what good manners are for.

It's not politeness, it's being a professional!

Despite what you might read on LinkedIn, it is absolutely not necessary to bring your authentic self to work.
Autistic or not, part of life is having to rub along next to people that you don’t like. I doubt there are many people who find this easy all the time.
It’s work, you just have to deal with it and get on with your job. No need to be best friends with your colleagues, and no need to be rude either.

Edit to add: stop using neurodiversity as an excuse for being poor at professional relationships. Do you really think that all neurotypical people are always being their authentic selves? Of course we are not! As if I want to hear about half the stuff I have to listen to in my professional and personal life, but that’s just what you have to do sometimes, unless you don’t mind people thinking that you’re an AH.

Noise cancelling headphones are god's gift to the office in my view. No better way to cold shoulder people you can't go beyond "civil" with.

This. It's part of your job to behave in a socially acceptable way to your colleagues, I'm afraid. There are some jobs where this is less important or onerous than others.

When she comments on your messy desk, just say ‘I like it like this’. That’s not hard and doesn’t invite drama of any sort. To your wider point, everyone NT or ND, has to adopt a work persona (or mask, if you like) and hide their true feelings about annoying colleagues.

This as nothing ti do with autism and everything to do with you being rude. Please do not use being in the spectrum as an excuse ti not understand general politeness. You are in employment so its safe to assume you are quite capable of this as it sounds like you work in an office.

With the desk comment, why do you think she is saying it?
Sometimes comments like that can be subtle bullying, other times they can be just a genuine thing. Maybe the person has a form of OCD and struggles with messy desks for example.
If you think its meant as a put down, Id say something like 'you seem to be very focussed on my desk, do you need some extra tasks to do?'
If she genuinely is a germaphobe or similar Id probably make an effort to tidy the desk a bit on the other hand.

How do you distinguish between being authentic and being unnecessarily blunt?

You don’t. I can’t stand the type of people who say things like ‘I speak my mind’. Learn to hold your tongue and don’t be rude to people.

Sounds like Basil Fawlty tbh.

How do you work alongside people you genuinely dislike without feeling you're constantly pretending?

• I am there to be professional and get the job done. Getting along with people and finding the "good" in them is part of my job. It will also help me progress in my career. I can work with people who are very different from me and focus on their strengths and the good things they bring. I expect that's why I am highly valued at work and don't struggle to get promotions

Is it acceptable to be polite but emotionally distant, rather than friendly?

You don't need to be best friends but most people have something great about them if you take the time to get to know them. It's grim working in a cold and distant team. Remember you are building your professional network.

How do you set boundaries with colleagues who irritate you without creating conflict?

• I tend to just let it roll off me if they are a bit irritating. I expect I irritate people too.

How do other autistic people cope with the expectation to filter their genuine opinions at work?

• you need to be professional. I am assuming you would manage not to be rude to a customer. I would also try and see things from other people's point of view

How do you distinguish between being authentic and being unnecessarily blunt?

I always think about how what I say might land with the other person
I also remember they might be fighting all kinds of battles I don't even know about

I am there to get the job done and be part of creating a team that works well together.

As PP have pointed out, these dilemmas are not confined to the neurodivergent. They're part and parcel of working life for many people.

Also, I thought having a messy desk was just "who I am" for years. Then we moved to hot destiny and have to clear our desk every day. It turns out I am perfectly capable of doing so once I make it a priority. I just need someone to make me do the same with my house now

To make it more simple just think of it like this: part of your wages are paid for you to be polite, friendly and professional to your colleagues. If you can frame it that it's part of your job and you just have to do it then it's probably much easier to do.

No one can be 100% authentic themselves at work.

It's one of the things ND people seem to claim the right to without accepting we all have to be professional grown ups at work not focused on ourselves but focus on work.

My authentic self is at best unemployable, and at worst should not be out unsupervised.

I would just smile and nod.

i really dislike my colleague's attitudes,
luckily i let it go over my head while they bitch

I think this is good advice. I wouldn’t want to hear about someone’s handbags, either. It would bore the tits off me. But civility is a part of most jobs. Think of it as teamwork experience.

You have to wonder if he is depressed as well. His world view seems to be very negative. You have my sympathies, it must be hard living with him.