Granted, I’m not sure if such measures were necessary here, but it is a major family secret, so the paranoia and least makes sense from a story perspective. My question is what exactly they were planning to do if he failed the test. Like, were they planning on to give him a second cup of tea? He already thinks you poisoned the first cup, so why would he accept any other drinks?

He'll forget everything in the last 72 hours so i doubt his consent matters, they could probably just pin him down and force it down his throat.