In Ex Machina, If Caleb had already reprogrammed the security protocol, why did he attempt to get Caleb drunk again? Or perhaps Caleb tried to get Nathan drunk again because he thought that Nathan had been listening in on his conversations with Ava during the power outages, and thus Nathan would have been expecting Caleb to try to get him drunk, and to not do so would have made Nathan suspicious and possibly led to the whole plan to free Ava to be foiled?
It seems as though you answered your own questions, and that's the answer I was going to give: Caleb knew (or suspected) that Nathan had heard his conversations with Ava. If Caleb hadn't tried to get Nathan drunk that morning, Nathan would have known that Caleb ALREADY completed his plan, and he would have undone the computer code that Caleb altered.
According to my thoughts, maybe there was another purpose. Although the security protocols are changed by Caleb before, it wouldn't be easy for Caleb and Ava to escape when Nathan is awake.For sure, Nathan would have been tried to hold them with his full effort. So, if Nathan gets drunk and Ava triggers a power failure, then they could have escaped early without any resistance. Mind that, Caleb told himself that he was suspecting, he was not sure.