I don't see how any of this contradicts anything in my post, so I have no idea why you have answered "In fact, no".
For a start, you are acknowledging there is no mechanism to force an individual to leave upon request, or compulsion upon the individual to actually comply with a request to leave, and yes, if the police attend and decide an offence has been committed, of course they will act. This is no different to the situation as is, or as it has been since 2010.
There is a world of difference between simply using a loo for it's intended purpose, and conducting yourself in a manner which contravenes Indecent Exposure legislation. Given that women's loos involve cubicles, in order to glimpse the penis of a man simply using the toilet for it's intended purpose, you would, presumably, have to be either looking over or under the walls/door yourself. That being the case, I don't fancy your chances much of convincing the police it's the man who is one committing an offence here. FWIW, simply having a penis on display is not, in itself, ordinarily enough to cop an Indecent Exposure charge, hence why men who are caught surreptitiously peeing in public are not, ordinarily, hit with an IE charge. There is normally some sexual element to the exposure when IE comes into play, and simply holding your penis while you pee is not a sexualised behaviour. It's also likely any man in a cubicle would be peeing sat down, so possibly not even holding their penis in any case.
Of course if someone is waving their penis around in the communal part of the loo, then that's a different matter altogether, but then that's not in any way just "using the toilet for it's intended purpose", and any man behaving like that in any space is already committing an offence, the fact that we're discussing a loo scenario here is rather moot.
And you KNOW that a mouthy TRA is going to make a scene DAY ONE. And force an arrest
I suspect you are correct, and it likely will result in a rumpus sooner rather than later, however, I also think there is potential for this to occur in not quite the way you specify. "Day one", when is that precisely? Post-EHRC guidance? Again, that will not change any laws, so even if it emboldens people, they are still going to have precisely the same authority they have now, i.e. none whatsoever. I think it's also likely that there will be an instance of a biological female being challenged, and that will also lead to a kerfuffle of some sort, so the status quo isn't particularly workable for anyone, in any respect.
Just FYI, I voted YANBU in your OP, so don't for a minute mistake me for someone who is all for a free-for-all in spaces. I'm not of that view at all. My only point is, and always has been, that the SC ruling and EHRC guidance are going to make no appreciable difference to the status quo because no laws have been changed and no additional laws have been made. You are entirely correct in that if the SC clarification is going to be enforced in any meaningful way, then there has to be some new law made to bring that about, or at the very least, a change in current law, because as things stand the clarification in and of itself changes nothing beyond the fact that organisations now have clarity on what "single sex" precisely means for the purposes of EA2010.