As you said in point 3, mental health diagnostic criteria are a tool for diagnosing and treating people who experience distress from their cognitive process. Doctors and psychiatrists exist to give medical advice and treat people who need help, not to tell them how their brain works out of curiosity. On that point though, depression and anxiety are two completely different things, I've experienced both (separately and simultaneously) and they're distinctly different in how they feel and how one would describe the symptoms to a doctor or a friend.

I went to the doctor in my mid-20s to get treated for ADHD, as it was causing me significant problems in my life, having gone through bouts of anxiety, depression, and suicidal ideation for the year or two prior, which the manifested itself in an inability to manage the negative traits of my ADHD. I've had it all my life, but only realised I likely had it a couple of years prior, but it was never something that caused big enough problems to go to a doctor about, mostly just report cards stating "easily distracted and distracters others".

At the same time I was diagnosed with ADHD, the doctor said it was likely I also had high functioning autism. I guess maybe I have it, but it's not something that's ever caused me any particular problems in life, so the doctor basically said not to worry about it and only to seek treatment (therapy or counselling) if I felt like I needed it. Same story for the ADHD too, he said there was no need to continue seeking treatment if I felt like I didn't need it at any point in the future.

On the other hand, being put on medication for ADHD (clonidine and dexamphetamine) has made a world of difference for me, and I've seen vast improvements in quality of life. Most importantly though has been recognising and accounting for the fact that my mind functions in a different way than most people, and making accomodations (and asking others to accomodate) for this.

Interestingly enough, he also mentioned the possibility of having bipolar disorder as well, but said that it was likely the symptoms were actually ADHD masquerading as bipolar and decided to treat the ADHD first and see if the symptoms remain. Diagnosing mental health issues is a difficult task, a lot of people with ADHD are misdiagnosed with bipolar, or people with bipolar diagnosed with clinical depression. If it wasn't for the fact that I was fairly certain that I had ADHD when I went to the doctor, I wonder what I would've been diagnosed with.

I think the language around mental functioning and disorders isn't great.

If you have Asperger's or ADHD and don't experience distress or difficulty in life from it, then it's not really a mental illness or disorder, it's just a different way your mind works. ADHD has a particularly bad name, it's not an attention deficit to begin with, but rather an inability to chose what your attention is on (you should see me if I'm in a hyperfocus/flow state, it's definitely not a deficit of attention), and it's not always hyperactive. I have primarily inattentive-type ADHD, you wouldn't pick me as having it just from looking at me, I display barely any "typical" traits of ADHD, it's mostly mental for me (mood swings, inattentiveness, impulsiveness, poor working memory, no concept of time, rejection sensitivity and a lack of emotional permanence). I don't think it's intrinsically a disorder either, it's been hugely beneficial in parts of my life, and I don't really want to be labelled as disordered or broken for the entirety of my life, it only became a disorder at a certain point in my life, largely due to external factors at the time. Often it's helped me function above and ahead of neurotypical people.

I know plenty of people with ADHD who don't take any medication and don't feel the need to, and get by perfectly fine in life. My father very likely has ADHD (it's hereditary, and he displays a lot of traits), and has never seeked treatment for it, and he's a very successful man. Same for people with bipolar, autism, or other neurodiverse minds.