This is a solid and constructive answer and I'd love to go into it in detail but I won't have time. I'm afraid I'm not going to be able to do justice to it, but let me try.
> So in the absence of society you would not be able to physically distinguish between these two gentlemen [1][2] and would classify them as the same ethnicity?
They have different phenotypes, and are clearly distinguishable visually. Now, if they were brought up in the same place (say in the same town in the US), and both met a blind person, would the blind person be able to tell the difference?
Some white people choose to turn a darker colour with a tan - if they could go completely black, would it matter? (other features wouldn't change of course, but you see my point).
That's not a good answer, so let me try a different one. To me ethnicity matters because people choose to make it matter. The difference between a cis- and trans-woman does not matter to me, but it seems to matter to you. These things matter only if you choose to let them matter. And a few weeks ago I asked a trans girl out because I like her (answer: no. As bloody usual).
> Did he receive any LEGAL benefits normally given to large predatory felines? If no, then intervention isn't required. If yes, then absolutely I would oppose it.
More likely, did he lose human benefits and be eligible to be shot or put in a zoo? That's a good point, well taken!
> Elizabeth Warren [...] "internal" identity for legal gain
Legal gain? Didn't think she got anything except some vague cachet due to american indians being kewl. And because that is IMO crap because such identity is meaningless to me, she gets no cachet from me and IMO should get none from anyone else.
> Rachel Dolezal
Yep. But pretending to be black only mattered because it matters in US society.
> Both demonstrate that these aren't really hypothetical issues, but present-day issues crossing the spectrum of not only social issues but also medical and cosmetic technology (elective, plastic surgery, skin treatments, etc...).
True. And maybe I'm sidestepping the issue, but it only matters because people think it matters. Being a cis- or trans-woman only matters if people care. I don't, and I don't see why it should to anyone else (I'm not saying it shouldn't, I just don't see why it should).
> But honestly, if I were to elaborate on my position it would be a long, kinda-rambling mess, so I'll table that point of the discussion.
My position is as above - I don't care - but it would be great to sit down and talk with you.
> Every human is operating within the restrictions of their physical form [...]
Good point.
> Short guys experience all sorts of emotional abuse and nobody really gives two shits about them either
Actually I'm surprised at that. Once children grow into adults, they usually judge people more on what's inside than out. But again, it's what society thinks, and if it didn't care, it wouldn't matter.
Now for some easy ones:
> Should my friend declare that he is 6'1" because he FEELS taller?
No
> Should we update all of his government documents and forms of identification as such?
if he got surgically taller, yes, otherwise no.
I think you point is if a bloke feels like a woman, is he immediately one? I don't know. If he got surgery + hormones and transitioned, then yes.
But I can elaborate on that a bit. One person I know is becoming a guy. She prefers to be called 'he' but recognises that they don't look like one so doesn't mind when 'she' slips out. As they look like a woman but prefer to be referred to as a bloke, I will do what she prefers, but you would, or you wouldn't? I'd call it bad manners to go against their wishes and hurt their feeling.
Someone I knew referred to a very effeminate guy as 'it', as they weren't in his view male or female, so: 'it'. That pissed a few of us off.
It's about feelings. So to your next point...
> Feelings are ephemeral, fleeting, [...]
I disagree. Briefly, I had a large chunk of my life wrecked with clinical depression. That's pure 'feeling' gone wrong. YOu cannot imagine how much that hurts. Feelings are the core of our being, that distinguishes us from philosophical zombies https://en.wikipedia.org/wiki/Philosophical_zombie Feelings are all their is that matters, because without feelings, there is nothing. We are computers.
> No. Life is hard. Get harder.
Many years ago I knew a woman who lost both twins at birth. Suppose that tragedy (she was suicidal. BTW nothing ephemeral or brief about that) could have been avoided medically, do we do that, or do we say to her "toughen up" and let them die?
Obviously we change what we can for the better. Centuries ago we lived with arthritis, gout, migraines... now far less so because we can do something about it.
So why should the feeling of being born into the wrong body type be different? See sysbin's answer for why (IMO) it isn't.
OK, I'm not going to take this much further because this whole subject is frankly going to take a day of discussion for both of us which don't have. I do not think you're wrong, I'm just trying to understand your view, and my answer above is... not good, I appreciate that, but it's my best shot. BTW, I really did find your answer measured and thoughtful.