#actually advice

So for context I'm trans non-binary and at some point am going to legally change my name. To a ridiculously long version of my name I use because sue me my legal dead name is long af and I love the length to be able to gauge how much trouble I'm in by how much of it is yelled.

I get along well with my parents (they've some what apologised and are better than they were a year ago) but they're also the main reason I have so much repressed trauma it's physically harming my body so...

I also just see them as people I'm biologically related to with no real deep emotional connection to them and all having a surname that is the name of a major district area SUCKS.

While we have very limited experience with the interaction of parents and system members (the parent we came out to was incredibly closed minded about it) we do have experience trying to convince people that we are, in fact, people.  I'm going to guess you're asking because you've already come out, but they're treating your others like a problem, so this response functions under that assumption.

Our first suggestion is to have one of them speak to your parents; have them introduce themselves, and make sure it's someone friendly or warm enough that they won't make your parents feel a need for defensiveness or aggression, maybe someone who's had a good history of dealing with your parents. 

If that doesn't work, you, the one who they view as their child should write them a letter, or an email, where you explain in depth exactly what your headmates are to you; what they've done to protect you and how they're separate people, not just symptoms or a neurological blip. Make sure you get it all down, and give them space and time enough to read it if possible, without you there so they can process and discuss between themselves. Do something nice for yourself while you do. 

You can always direct your parents to this blog (and we can hide or delete this post so they don't find your blog, if you'd like to) and beyond that, we suggest having a look through AstraeasWeb's FAQ and seeing if that's information you would want your parents to have on plurality. Expose them, if you think it will help, to blogs or videos of alters. This might be the right situation with which to enter into a discussion about watching the United States of Tara together, or something of the sort. There's also the Layman's Guide, which, while a bit outdated, is still a great resource.

be diagnosed. Now, I did not think by any second that my friend would lie to me about such a thing, and his alter has become a wonderful friend and a big part of my life now, so I tried to research on /why/ is it needed to have at least two alters to be diagnosed. Do you guys have any idea about that?

I don't think you need more than one alter to be diagnosed; you need at least two separate identity states, meaning that if you count the host as one, a two-alter or diapathic system is absolutely legitimate by DID's diagnostic criteria. 

An amnesiac barrier between at least two identity states is required for diagnosis, as well as the general presence of another identity state, meaning that the wording is often skewed to mean that there must be at least two identity states beyond the host; I've never met anyone who specifically called out diapathic systems as illegitimate, nor anyone who denies diagnosis on the grounds of the number of others present unless they were severely mistaken on the nature of DID in general.

Even then, DID is not the only explanation for multiplicity! Non-DID plurality information is a viable resource to look into as well, and your friend may not know if he has additional alters; many alters are known to show themselves after years of playing on and off with the body.

I'm glad you support your friend so passionately, and that you were intrigued enough to come to us for help. It means a lot to see a singlet supporter of that caliber.