Question: Given that the whole break between Redford and Pitt is over Redford's willingness to sacrifice people [even Pitt's girlfriend is given to the Chinese] what explains Redford's sudden change of heart and willingness to risk all to save Pitt AND his girlfriend?
Answer: Muir has become frustrated with the CIA as a whole - now carrying out bugging ops on trade talks so the US can sell more "toaster ovens", as opposed to the 'righteous' work he's done all his career. This is compounded when he sees that the CIA are prepared to let a man (Bishop) be killed over it, which also cements his determination to stop it. Although he has underestimated Bishop's feelings and determination to rescue Hadley, that's not really the point. He sees that Bishop is doing something righteous, 'the right thing' so to speak, and sees rescuing Bishop and Hadley as a chance to a) do something worthwhile again before he retires, b) redeem himself for past sins (sacrificing people/Hadley), and c) poetically stick it to the CIA - using their own resources to carry out the rescue mission amidst the trivial 'work' they're doing now. Muir knows that when he retires he'll be broke, but he'll be able to live with himself - he did something that was morally the 'right thing', as opposed to being the cold calculating operator he's been all his career, with that conditioning being another reason he now resents the CIA (he doesn't like what he's become). He's kicked against the bureaucrats who are doing all the wrong work for the wrong reasons. Also, there's a parallel between Bishop's feelings for Hadley and his attempted rescue mission, and Muir's feelings for Bishop (he loves him like a son or a star student). Muir certainly sees Bishop as family, and you don't mess with an Amercan's family. Muir says as much in the film with the analogy about his uncle's plough horse. The student is now teaching the master - about doing things for the right reasons (love and loyalty as opposed to death and toasters).
Answer: Several reasons: Muir felt no loyalty to Elizabeth Hadley, but he did to Tom Bishop. Muir had faked a letter to make Bishop believe that Hadley had ended the relationship, so he was not supposed to know that she was in Chinese custody. Muir had underestimated Bishop's feelings for her, and his capacity to work out the deception and attempt to rescue her. Also, Muir believed that Hadley was a threat to Middle East ops, however as he was now retiring from the CIA he no longer had responsibility for this area.
Chosen answer: He might have taken a medical equivalent to a regulated dose of Curare, which can suspend the body temporarily in a coma-like state, while keeping the mind active and perceptive. It could also have been tetrodotoxin, which has remarkably similar effects, but lasts longer. In either case, they were specifically engineered for his physiology, indicating a pharmacologist's aid in their endeavor.