Regarding the WI, the problem for me is that they are using the word “women” as if it meant “people with woman gender” and, simultaneously, biological women. That doesn't seem right to me.
The Sex Matters guide says that having “organisational policies” not based on the EA’s definition of Sex is “likely to result in unlawful conduct”. I think that applies here.
On the question of whether any random group of more than 25 women could decide that their organisation included women and MCW and excluded men who didn't claim to be women, I think the answer is that that would be unlawful. I think that the premise of the EA is that no-one with a PC can be excluded unless the exceptions apply. On the flip side, it would also mean that if you include men, you couldn't exclude MCW. And you couldn’t include gay men but exclude straight men.
Anyway, IANAL, so...
Regarding whether GII would pass the Grainger test, it's not about revenge, obviously.
My starting point is that GR as a standalone PC is not working. It's ill-defined. Nobody knows what it looks like. It protects fetishism as much as it protects people with dysphoria, etc.
So how do we protect people who claim a GI from discrimination?
As a belief, I believe is the best way. Manifestations which don't impinge on other people would be fine but others wouldn't be. This would stop the fetish aspect - turning up to work or the PTA meeting dressed in fetish gear, or engineering wardrobe malfunctions. Specific manifestations of belief could be questioned. It would be treated as we treat religion - reasonable accommodation made.
That's why I was interested to know if some beliefs might pass the test while others failed. I wanted to know if a court (or anyone else) would analyse GII as a belief or set of beliefs. Does it have to be taken together in one lump? Does it pass if most of it passes? Do some beliefs and not others pass?
I see problems even with the GII lite version, but I think that is more likely to pass. However, it's pointless to say GII shouldn't be tested. It will have to be, sooner or later, because it's such a huge influence in society.