> The mentats are theoretically safer because of their imperial conditioning, but even that can be tampered with, as we see with Dr Yueh

Dr Yueh was not a mentat, but a suk doctor who was subject to conditioning. (Which was broken by the Piter de Vries, via the pain amplifiers applied to his wife, Wanna.)

Paul himself was being trained to be a mentat, and there were no hints of conditioning there, neither with Paul, Hawat, or de Vries (albeit he was described as a "twisted mentat", whatever that means).