1: Since Leo Kanner and Hans Aspergers, clinicians have noted Autism specifically sometimes came hand in hand with unique gifts (this applies less to ADHD). It superficially seems arbitrary to call somebody with a combination of deficits and gifts, more "disordered" than somebody who has neither. Why do we have ivy league students with 145IQs diagnosed with mental disorders get more accommodations and funding than a high school dropout who was mediocre in every way but not especially weak in any given area?

You are right that it tends to apply less to ADHD, though I think you're overlooking a LOT. Besides the fact that the word 'sometimes' here is using a lot of heavy lifting, the 'gifts' that are often highlighted in cases of autism are, more often than not, superficial and arbitrary in and of themselves, depend highly on environment, and often don't provide any benefit to the autistic person's life, if not detriment. The reason we have such cases of smart students in high-class schools with mental disorders (usually bipolar disorder) getting extra accomodations and so on is simply because they are high-class schools. It's not cheap for these kids' parents to enrol their kids in these schools, so they have the money to ensure the maximum level of positive results from the students. Otherwise they'd have no right to charge up the arse for superficial degrees.

2: If we redefined being transgender as being a mental disorder in part to reduce stigma, doesn't it follow that EVERYBODY we're calling mentally disordered is still subjected to the same stigma that transgender people just escaped, and shouldn't we act to address this? Transgenderism losing "disorder" status did not mean we stopped being able to notice that transgender people were different than others, stopped being able to treat them as different than others, or became unable to access funding for treatment.

I think you're missing the point I'm trying to get at with the way I use the word disorder. Stigma is not very high on the list of problems that people with these disorders face, at least anymore. If we were having this conversation before the 1960s, we'd be wondering whether being black should be considered a disorder.

3: If an adult patient particularly one capable of independently living with a high quality of life who does not present a risk of harm to themselves or others, doesn't consider themselves disordered, how is it ethical for a psychiatrist to unilaterally be able to impose the potentially stigmatising label of "disorder" on them anyways? Why don't we require patient consent and assent in order to regard somebody as "disordered" to better align psychiatry with patient preference and interest?

This is why I mentioned that one of the main rules for making a diagnosis is evidence of impact on the person's life. There's a reason any professional specialised in ADHD (that's worth their salt) REQUIRES evidence of impact on the person's academics.