Theory 1, Coincidence: Trump was pro-Russia and a number of pro-Russian operatives of various degrees of sleaziness worked for him, but their shady dealings were all totally unrelated. Certainly there was no actual Russian influence on the Trump campaign. This seemed far-fetched at the time, and given that we know know the Russian government had intermediaries approach the Trump campaign and make clear that the Russian government was pulling for Trump, I think we can safely rule it out.
Theory 2, Birds of a Feather: Trump's pro-Russia views made him unacceptable to any establishment conservatives, so a bunch of shady pro-Russian operators flocked to him instead. Trump, desperate for any team he could assemble, didn't ask too many questions about their backgrounds. The Russian government, presumably because of all these pro-Russia players, was running interference for Trump, but the Trump campaign had nothing to do with it. I believed this once. Given that actual Russian agents approached Trump's very inner circle and let them know of Russia's support sort of rules this one out.
Theory 3, It Wasn't About Trump: The goal was purely to hurt Hillary; any benefit to Trump was purely incidental and certainly not because of any pro-Russian operatives in his campaign. Again, ruled out by Russian intermediaries telling his very inner circle that Russia was pulling for him.
Theory 4, OK, There Was Russian Infiltration, but Trump Didn't Know: This view would see Manafort and/or Page and/or Flynn and/or Stone as Russian agents carrying on nefarious activities without the boss knowing. OK, I have to concede that the presence of active Russian agents in Trump's camp remains speculative. But we do know that now that Junior, Jared and Manafort all knew that Russia was aiding them. Maybe Junior and Jared were so naive as to think the Russians were doing it out of the goodness of their hearts, but Manafort must have known they would want something in return. And the only way for Dad not to have known what his own son, son-in-law, and campaign manager were doing was a deliberate effort to shield him from what was going on to allow plausible deniability. Even this theory is starting to look too innocent to meet the facts.
Theory 5, Trump Knew or Should Have Known: Trump was willfully blind to so many people's extremely sleazy ties to Russia. Well, duh! And, again, willful blindness is the most innocent thing to suppose about the meeting.
Theory 6, Kompromat: The Russians have something on Trump to force him to do their bidding. Whether the Russians had anything at the outset of the campaign is a matter of pure speculation. But they sure as hell do now. Over time the collusion itself becomes the kompromat. If this didn't happen at the June 9 meeting, it seems safe to assume that it happened shortly after. Of course, that begs the question of why the Russian government wanted Trump to win in the first place.
Theory 7, Trump is a Russian Agent: This is so mind-boggling that the authors refuse to take it seriously. Yes, it is consistent with the facts. It seem inconsistent with Trump's character. He is a total loose canon who couldn't keep his mouth shut if you sutured it, and he doesn't want anyone to tell him what to do. Doesn't seem like the sort of person anyone would want as an agent. But I suppose you have to take the agents you have, not the agents you wish you had.
At the time, the authors commented that theories 1 and 7 seemed implausible, so they believed the plausible answers were 2-6. It seems to me that by now 1-3 can be ruled out and 4 is giving Trump way more benefit of the doubt than he deserves. That leave 5 as the most plausible theory, 6 as the second most plausible, and 7 as -- well, I am still willing to give Trump the benefit of the doubt to that extent. But not one inch more.
(PS: New label now, "Trump and Russia")