Frankly, I think what many are ignoring here is that there are deep state Republicans and Democrats, and that both groups wanted to bring Trump down. There is another faction of people who didn't like Trump anyway, and supported a different GOP candidate early on. I think many of those sat this one out, or did not actively oppose Trump to the point where they would push Hillary, just did not support him either. Effectively, an abstention is still a vote, and the Third Parties bagged a lot of interest as a result (from people who wanted neither Hillary nor Trump and voted that way rather than stay home).
Back to the GOP, though, if we look at the usual suspects when it comes to voting with the Dems on critical issues in Congress, (McCain, Collins, Graham, Murkowski, etc.) the first name on the list is none other than McCain.
Whether motivated by envy, politics, some altruistic defense of 'murica, or the random backscatter of impulses in damaged neurons, I won't speculate, but the bottom line is that as a prominent pol, his handing such a document over would give it a credibility it might not have had otherwise, and that may well have been part of the deep state plan, whether McCain was fully cognizant of that or not.
I won't waste time judging McCain, The Almighty can handle that without my imperfect input.