Wow - I expected fast-moving, but...! Now I find I can't resist engaging when I've really got little time to. But thanks to everyone, especially Trying, for engaging with my question in such detail.
Perhaps inevitably, the pages since I asked it evolved into an argument about the definition of woman. Perhaps 20 years ago - maybe even 5 - this term was, effectively, uncontested: it was universally accepted to mean "adult human female". Recently, the definition has been problematised: as the unifying feature of the original class the noun "woman" described was biology, thus excluding all men, proponents of the new class argue for the replacement of biology with gender in order to include some men. This shift requires the introduction of a series of sub-classes to accommodate those who belonged within the original group, for example, "cis".
I find it interesting that the debate's remained largely semantic, as opposed to addressing the real-world implications of this change, which is what I wanted to encourage in my original question (the quite genuine increase in relative risk to lesbians on a dating app hasn't yet been addressed, for example). This semantic debate will become circuitous, as it has, because the truth of the matter is that we have two groups who both feel invalidated or negated by the retention of the first definition (offensive to many transwomen) or the introduction of the second -including the associated use of prefixes such as "cis" as the need for these in itself is an acknowledgement of the primacy of gender and relative irrelevance of biology in the revised definition (problematic for many women). How do we negotiate this? Simply defining words and offering hypothetical subsets and Venns just leads to circuitous discussion.
For me, it has to come back to the real world applications of the word "woman" - its impact on actual lives (ditto, "lesbian"). I do feel this is something you, in particular, are not addressing, Trying. I'd be interested to know what you think about the following.
Your well-intended question, "To help me in the future, what is the appropriate term for non-trans women on here?" to me lies at the heart of the issue. It shows that women have, in perhaps a decade, lost the ability to describe themselves as a political class with unique rights. And they only recently gained this right (and by recently, I mean, in some instances, the last few decades or so): think of property laws, bank accounts, marital rape - all of these relating to the simple recognition of woman as an authentic human being in her own right as opposed to defined in relation to - and as the chattel of - her husband. Can you see how attempts to redefine woman, a word key to the current legal protections of and recognition of an oppressed group, may terrify and infuriate, members of that group, Trying?
If "terrify" and "infuriate" seem disproportionate, then consider the real-world effects of this recent redefinition of women - real-world effects that others have referred to in this thread. Rape crisis centres (a recent service, hard-won) are calling single-sex care and psychological trauma bigotry. Hospitals are being instructed to reject a distressed patient's fear of a male body on the ward as bigotry - even in the aftermath of rape by said male in one instance. There's currently earnest debate among politicians about the validity of female sporting categories - sports that were forbidden to women perhaps a hundred years ago, then existed for a precious few decades for the sole purpose of identifying the strongest, most skilful female bodies, being accessed by male bodies whose inclusion would eradicate their original purpose. Medical research and systems which have neglected women's bodies for generations is being corrupted by the inclusion of this new category of women. There are examples of all of these. And they aren't niche - they're being embedded into the fabric of our society as I type. Because of - enabled by - the redefinition for which you argue.
We can argue all we want about sets and subsets, but is this realistic or even fair if we acknowledge that the argument itself is, by very definition, in its real-world consequences, an argument for diluting the hard-won rights of an oppressed group. This isn't what you want, or believe, but it is what's happening. As long as this isn't being recognised (and if you yourself don't really seem to, if I'm honest, even as someone who's engaging to the degree you are, and debate is being shut down as bigotted, then what hope of open recognition on a larger population scale?!) isn't there an argument for stepping away from semantic utopias in which all distinctions are established and rights protected to fight against the very real degradation of women's rights that arguments such as yours, for these ideals, are, I'm afraid, legitimising?