No, I didn't ignore that. I specifically said, "because she teased some trans advocates for drawing a bad parallel it negates all that?"

She doesn't expand--in the source you quote--on what she thinks it means for humans to be a dimorphic species. Given that she talks about cases using the standard language that acknowledges exceptions, one could charitably imagine that she admits exceptions to the rule. Given that she says the word dimorphic, one could uncharitably imagine that she insists on a strict binary with zero exceptions. (Some people do.) If you want a proof-like argument, you need to take the charitable view; if you just want to show that it's plausible, you can be somewhat uncharitable.

Keep in mind, though, that biology is messy. Humans are bipedal. Except some people don't have legs, some people have motor dysfunction that prevents them from coordinating walking, etc. etc.. This doesn't mean that if someone says "humans are bipedal" that they think that people who can't walk shouldn't exist. It just means that there's a pretty common characteristic and it's handy to name it. Likewise with sexual dimorphism. The United States has a two-party political system...this doesn't mean that third parties should be outlawed, just that they typically aren't very prominent.

And again, she didn't actually say that trans people in bathrooms is dangerous to cis women. What she said was that allowing people who are fully morphologically male, and look it, in bathrooms is dangerous to women. Every single time, she says something like if no hormones or GRC or anything is needed, "then you open the door to any and all men who wish to come inside". She is saying that men are dangerous to women. She might be saying that trans women who look like men, have no surgery or hormone therapy, and have not been evaluated by psychologists, are not women but are actually men and/or are themselves dangerous to women, but even that isn't clearly stated here.

The only thing that's clear here is that she views behavioral norms around perceptually obvious gender identity as a layer of protection for women against men.

The costume thing is a better point, though JKR never actually mentions clothing. That's you. It's a plausible interpretation, though. (I didn't talk about this before because I thought your original piece did an okay job of making a case there; not a very strong one but it was neither the worst nor best case so I didn't mention it.)

JKR doesn't argue in her piece that trans women aren't women at all, though. She says that she objects to the attitude that, "Women must accept and admit that there is no material difference between trans women and themselves."

That is the context in which she makes the costume comments. She's not even saying that trans women aren't women, necessarily (but then again she never comes out and says they are). Rather, she's saying that she feels that there are important differences. We can infer from her comments that she feels that at least to a large degree femaleness resides in having a sexed body--this is fundamental--not in whether you superficially conform to some sort of outward-facing norm (whether this is literally clothing, or it is being-feminine).

So, suppose that to JKR, the experience of being female is tightly tied to having a morphologically female body. She says, "I remember how mentally sexless I felt in youth." Is that wrong? Does that mean she's not a woman--is she experiencing womanness an invalid way? (Is she lying? Why would she?)

Having an experience (which sounds different in some important ways from yours) doesn't make one right, of course. There might not be a right, in that it might not be an objective matter; or one's experience might be misleading.

However, if you accept that this is her experience, then it makes sense that she would not accept that trans women are women with no material differences. She may or may not accept that they are women but with material differences--she doesn't quite come out and say. And either way, this wouldn't necessarily rise to the level of transphobia. One could have this experience and be perfectly okay with trans women, but still especially value the shared experiences with other natal women of having a life in a sexed body. That seems pretty reasonable. We don't insist that het guys talk about how hot other guys are with gay guys or the het guys are "homophobic".

So I don't think this is a very strong case. A cause for alertness, yes, especially since it's very easy to promote what ought to be an understandable preference into an iron curtain that institutionalizes mistreatment. Hence, my assessment that it is of middling convincingness.

I do, yes, over and over, advise that if we want to have a strong argument, we either need to take people at their word, or have very solid reasoning for why we mustn't. Otherwise it's an argument about plausibility, not about what any sensible person should believe. I'm assuming the latter is what you were going for by calling it "proof". If you only claim the former, I think you've already done a more than adequate job.