Trump had more than one opponent. Granted that the Democratic one was the only one likely to win. But those who voted for other candidates DID support his opponent. Just not the opponent you preferred.Its an imperfect world. Would be great if someone could wave a magic wand and make the problems go away (without introducing all new problems). But until that happens, anyone who supports a known bigot like Trump (or, alternatively, does not support their opponent) is morally culpable for any bigoted policy decisions that result.
So does increasing the number of third party votes.Yes, some states were solidly democratic or republican so an individual vote wouldn't matter. Of course, a voter should never assume anything, but even if a single vote was unlikely to swing the electoral college in Illinois, I still think there is value in having a higher popular vote count (even if it doesn't contribute to electoral college victory). It stresses to politicians the electorate is less willing to tolerate racism.
People who would normally vote Republican staying home (or voting third party) helped the Clinton and hurt Trump. People who would normally vote Democrat staying home (or voting third party) hurt Clinton and helped Trump.I am not really sure.
Legally I think its a non-starter. (I doubt legislation could be crafted that would be constitution that would limit ballots to 2 parties.)
I suspect that if such a law did exist you'd just get more non-voters and spoiled ballots.
I don't think it was apparent at the time that Trump would win. You mention sending a message that a racist will not be tolerated. I get that, and kind of agree with it. But that assumes that that is the only message to send and that that is how it would be perceived. Perhaps "no racists" wasn't the only message voters might want to send. Maybe they wanted to send both that (which could be inferred from any of the three significant non-Trump candidates) and "less government" (Libertarian) or "more environmentalism/social justice" (Green).