I never could figure out just why becoming human from the Shanshu would be any different than becoming human due to the blood of a Mohra demon. Human is human, Angel loses all of his vampiric strength, speed, and whatever, and Buffy would still have to spend as much time protecting him as she did in the execution of her job. And, no, I'm NOT forgetting that the Shanshu is supposed to be a reward for Angel helping in the Final Battle, I'm remembering that there just isn't any such thing as a "final battle" where Buffy and her friends are concerned. No matter how many Big Bads they manage to defeat, it's always a certainty that there's a Bigger Badder just over the next hill. It just never quits. There's no escaping the fact that Angel had his humanity, free and clear, and he let the Powers That Be con him into giving it up by saying that Buffy would die protecting him if he remained human. So, instead, Buffy died protecting Dawn instead. Good trade, Angel. Shrewd.
(Okay, my "I Will Remember You" rant is over for now.)
As for the Shanshu, it's my understanding that he was to receive that from the Senior Partners, which would seem to indicate that he would get it as a reward for helping THEM, and since THEM are evil, that means that Angel must be doing something evil, kinda like the whole Twilight story line, in order to get it. Come to think of it, becoming Adam to Buffy's Eve in the Twilight Universe just MIGHT have been the Shanshu, or at least as close as he's ever likely to get in the next few centuries.
Oh, I just got a wild thought. In some future Armageddon. Angel does the deed and receives the reward, but, like is so often the case, the reward isn't without its built in penalty. Instead of becoming a human, he becomes a 4 armed monkey and has to guard the Watchers' Library in Fray's time.
Joss, if you're reading this, feel free to use it in Season 10, no payment or even mention required.