On your first point, while it’s possible that there will be people who might want to transition regardless of whether they pass as the opposite sex, the letter is making the point that this is the current premise on which paediatric medical transition is based. The established idea is that trans people will pass better as the opposite sex if they haven’t gone through puberty.
There’s an interesting point on this made in the new US paediatric gender dysphoria report (p12) (linked upthread) where it acknowledges this aim of medical transition in children:
In many areas of medicine, treatments are first established as safe and effective in adults before being extended to pediatric populations. In this case, however, the opposite occurred: clinician-researchers developed the pediatric medical transition protocol in response to disappointing psychosocial outcomes in adults who underwent medical transition.
I agree with you that other reasons to transition might become the dominant ones now that passing as the opposite sex is off the table. But the currently proposed PB trial would only have been based on current protocols and objectives, which is why this is the only relevant treatment objective here.
Re your second point about it being unethical anyway even before the SC ruling, I think the point the letter is making is that Sex Matters et al had already been arguing all those other points around ethics (as they say in the letter), but that this ‘you shall not pass’ argument is supplementary to all those other objections already put forward, in light of the SC ruling.