>Here's what I see, over and over again: Trump does something that is not exactly racist but ambiguous enough to be interpreted as such.
By far the most plausible explanation for that behavior is that he is in fact a racist. However, you've made it clear that you won't believe Trump is a racist unless he goes on TV and says "I am a racist". By your standards of proof, hardly any racists exist in the entire USA. It's also inaccurate, by the way, to say that birthirism is driven by partisan politics. The mainstream of the Republican party never touched it, and it was not a component of McCain's campaign. It's driven by people who don't like the fact that the current President isn't an old white dude. After all, McCain himself really wasn't born in the USA, but no-one ever made much of that.
My own preference for POTUS would be someone who, say, doesn't retweet bogus crime statistics by white supremacists, or promulgate factually inaccurate and highly incendiary stereotypes of Mexican immigrants as criminals and rapists. You may be ok with a POTUS who does those things. If you are, then there is probably not much point in us having a discussion.
By the way, it might surprise you to know that you also have confirmation bias. (As confirmation bias is something that we all have, it tends to be a bit of a double-edged sword, rhetorically speaking.) I can see that you are not going to accept that Trump is a racist simply because you interpret all the evidence on the assumption that he is not. However, the alternative hypothesis you're outlining is that he's someone who cynically exploits racial tensions for political gain. That is hardly any better.