By "horrible racists" I mean something like: people who denigrate[2] and/or work against the interests of some racial group(s) because they hate or fear them, or because they see the success of that group and the success of their own racial group as opposed to one another, and choose to harm the other guys for their own benefit.
You mean like all the Al Sharpton-style black demagogues? There are certainly a lot more of those people than there are white people satisfying your definition of "horrible racist".
Romney was presenting evidence of his viewpoint on Trump, and was expecting voters to trust his viewpoint as it was evidence of what a mainstream prominent Republican thought about Trump. How is that not evidence?
Yes, if you look at it that way it is indeed evidence. In particular it's evidence of Trump's claim to be anti-establishment.