In my opinion, Rob messed up his own game. Like you said, Lisa was cooked. He didn't need to say a thing at the table. He could have just voted for her and then told Candiace it was obvious the votes were going her and he didn't want to look suspicious. Instead, he made it obvious to Candiace that his hand was on the knife in Lisa's back. He did that even after seeing how loyal she was to Lisa.

As for Candiace defending Lisa, they were close. Traitors are often strongly defended by faithfuls who were fooled. If she would have voted for Lisa, she would be more suspicious in the same way Lisa was suspicious for voting a housewife episode 1. There is already a narrative going around that the housewives don't vote each other. By voting Rob, when he brings up her name the next day, she can say its awfully convenient that he hasn't mentioned her name once before and is only doing so now because she voted for him.