I'll step in for "Lawyer" here, Goldy, to point out that you get at least one detail exactly wrong in your drive to prove your thesis that McKenna didn't really win anything worth winning. You say:
Since there was nothing "nondiscretionary" about McKenna's actions, it is not clear that mandamus would be available even if the court found that McKenna lacked the authority to join the anti-HCR lawsuit.
In fact it's entirely clear that mandamus would be available. On p. 5 of the slip opinion, the Court cites a case from 1980, State ex rel. Burlington Northern, for the obvious notion that "The nondiscretionary act may be to undo an action unlawfully done." Let that be a small reminder that even bright people who deal in words for a living should probably grow a bit of humility and consult with an actual attorney if they want any level of certainty about what the law says.
Now what is quite unclear, which a hyperpartisan like yourself has no reason to report on and a nonlawyer like yourself probably missed, was whether the City of Seattle had any business bringing this case to begin with--that is, whether they were a proper party to sue. And there, it looks like Pete Holmes's office was making an almost frivolous argument that most lawyers would be embarrassed to file, that the city counts as a "taxpayer" but that it didn't have to follow the taxpayer lawsuit procedures prior to suit. The Court avoided answering the very simple "standing" question (which is normally considered a threshold issue), perhaps because they too are elected officials and they wanted this political hot-button issue to go away rather than simply inviting someone else who DOES have standing to sue all over again. Still, the Court's little procedural trick doesn't make Seattle's case any less embarrassing; it's pretty clear that Holmes instituted this suit as political grandstanding, since it was such a terrible legal argument. THIS is what a "stupid" legal argument looks like, much more than the arguments McKenna's office made in the Goldmark case. Not that I expect you to point out that a Democrat did something stupid or viciously partisan.
I get that hyperpartisan is your style, and that you're probably giving some of your readers exactly the preconception-reinforcing analysis they want. I get that you're not an actual journalist or lawyer who aims for unbiased analysis (getting past your own bias and seeing the strength of the other side's arguments is one part of formal legal training that you don't have the benefit of). But just in case you care about such things, you're starting to run the real risk of lulling your readers into a false sense of security about the Republicans' best shot at the governor's mansion in a generation. I'm a solid Democrat, but I'm well aware that Rob McKenna is not a garden variety WA Republican clown. Almost everywhere he goes, he is the smartest guy in the room. He is a good lawyer and an even better politician. Don't blind yourself to that, don't underestimate him, or we all may end up calling him Governor for years to come.