My theory on Trump's Oz endorsement vs Kathy Barnette:

Kathy has huge opp on the merits to win the general but there were some things in her history (especially tweets that would be taken out of context ) that she would have been clobbered with. Plus she might do better with the center leaning black population without Trump in the way. So they divide and conquer the problem. Half of Team Trump support her and the other half go against her to get all this out and have it put to bed (it was going to come out from D's and Rinos anyway) before the general. The Trump Oz endorsement makes sense (and it otherwise never has) in light of this scenario.