Passing is a red herring.
This is talking about entire classes of people, not individuals. The 'sweet little transgirl' situation: a MNetter described a situation a while back arising at her daughter's school. (Sorry whoever you were, I can't remember your name!) There had been one transgirl, shy, long known to the children, well liked by the girls who were protective of them, and they had no real issues about agreeing to that one male child joining them in the changing room. Which has its dodgy points, but the girls were willing to do the exceptional for this well known, well liked child.
This was fine until a creepy teenaged boy the girls did not like and did not trust with a history of dodgy sexual behaviour towards the girls, identified as a girl. They wished to exercise the precedented right of changing with the girls. The girls were not happy about this, but they'd agreed to the principle. The school had no means to allow the girls to choose who they felt safe to get undressed in front of: any male student who said the words got the right.
Third spaces. I am all for trans students being allowed to come out of their own sex changing spaces for privacy, dignity, meeting of their feelings of discomfort and concern, because I believe no one should ever have to be in a vulnerable situation and be expected to just cope with those feelings. But those students should be provided with a third space because everyone is entitled to have those feelings respected, and that is equally applicable to female humans.
Separate out the need. The need is for the trans child to have somewhere to undress that they feel safe and comfortable. The need cannot be to prove validation of the child's chosen identity by compelling girls to sacrifice their comfort and dignity with their bodies and by taking clothes off.
If you've chosen to change your body and appearance to the point that you may cause confusion or alarm in others then you have to take responsibility for that, and either explain or choose facilities accordingly. Although the very lovely TM I worked with a few months back presented initially as a smart, bearded and rather small and slight man until they smiled and said hello, and were instantly readable as female. As soon as I'd seen them from the back, the hips and body shape was equally apparent. I felt my body shift from the alert of working with an unknown male to the comfort of working with a female, I would have had no hesitation whatever in sharing a toilet or changing room with them because they were female. They were also a lovely person who I can't imagine would hesitate to help if they saw a female looking upset or scared.
I can't ever imagine for example them taking selfies in a female single sex space with a sword for example, and written threats of what they plan to do to females who displease them.