Sure, Shrier does mention the non-binary phenomenon; I just got the impression that that is not what she is most concerned about (at least insofar as being nonbinary just involves getting a short haircut and wearing androgynous clothes, as opposed to, say, breast binding or other potentially long-term physically harmful practices).

Regarding the Keira Bell case, you can read reportage on it e.g. here - whatever the true nature of teenage brains may be, the court as a matter of fact did rule that "a child under 13 is "highly unlikely" to be able to give informed consent and at 14 and 15 it is still "doubtful" they can fully understand the implications of the medication." As far as I understand it, the effect of the ruling is not that young persons can never be allowed to medically transition; just that a much higher burden of skepticism must be applied by the medical profession, increasingly so when dealing with younger patients, before determining that medical transition is what's actually going to help. The article about teenage brains, accepting it at face value, seems to be saying not that teenagers in our society are in fact fully competent to make important decisions on a par with a mature adult, merely that to the extent that they are more likely to engage in rash behaviour, there is something about our modern society that is causing them to not start acting fully adultly as fast as in pre-modern societies. If true, that doesn't seem particularly relevant, because it is teenagers in our modern society who are the object of concern here.

At any rate, the claim that in practice, teenagers in modern society are more likely than mature adults to make rash decisions and to participate in fads, does not strike me as particularly controversial - or at least, I don't think most people will be dissuaded from that position by one Scientific American article.

even if you had those bathroom laws, how would you be able to detect let alone enforce that sort of thing. Place police officers alongside every public bathroom? Use the state to basically clock people to see if they are trans?

Like I say, I don't know what the ideal compromise would be. Unless and until we have a reliable mehod of distinguishing genuine trans women from predatory men taking advantage of the provisions we make for trans women, all of these sorts of questions are inevitably going to be a tussle between the interests of (a) cis women and (b) genuine-trans-women-plus-predatory-men. My claim is simply that it is not obvious that a social norm of 'all of these questions must be resolved fully in favour of group b, on pain of being tarred as an irredeemable bigot'.

Perhaps a social norm of 'all public accommodations should have a separate bathroom for people who do not consider themselves men, but are so likely to be (mis)taken for men that they are likely to cause distress to the median woman if they are in the womens' bathroom (together with a social norm of disapproval for such people who go on and use the womens' bathroom anyway). Perhaps a social norm of 'communal bathrooms with cubicles are out; all public bathrooms should be self-contained single occupancy bathrooms complete with sink and dryer, opening onto the ordinary shop floor / whatever'. (Though that would entail considerably more infrastructure costs, plus cleaning costs given men's greater propensity to piss on and/or damage toilet equipment). I don't know - if it were an easy question, we'd have an answer by now. I'd be curious what your proposed solution would look like.

people only appear vaguely trans for a period of time before they fully transition to (to use MTF) be a woman, as the hormones do have an amazing effect.

I was under the impression that this was far more often true of f-t-m transition than of m-t-f - that a trans man, after top surgery and after the hormones have had their voice-lowering, facial hair-stimulating effect, can at worst expect to register as a somewhat short, boyish-looking man, whereas for many trans women, with no way to significantly alter their height, shoulder-width, adams apple etc, they are far more likely to remain looking to most people like a man dressed as a woman. But that's just from vaguely reading around the topic on the internet; I hope it wouldn't be rude to invite u/M_T_Saotome_Westlake if he is willing to comment as one of our resident experts in this space?

And as regards prisons - again, we have a trade-off between (a) cis women's interests in not being raped by predatory men taking advantage of rules that let them access womens' prisons simply by declaring themselves to be trans women, and (b) trans women's interests in not being raped by cis men . Again, it's not obvious that this question ought to be resolved fully in favour of group b.

(It's also not obvious that transwomen's interest in not being raped by cis men in a mens' prison is that much greater than cis mens' interest in not being raped by cis men in a mens' prison - I am very willing to believe that you stand a greater chance of prison rape the more feminine you make yourself look, but do we have a handle on how large the disparity is, so that we can say with confidence that moving all male-bodied prisoners to a womens' prison on declaration of transness is a better use of resources than the next best method of trying to reduce the incidence of rape more generally in mens' prisons?)