W&H would have created the prophecy if they were going to use it as a temptation to make Angel fight on their side in the final apocalypse, which is pretty much what the original President of the company told Angel when he came back from the dead (I can't remember season or episode, but maybe someone else here will. He told Angel that he would earn the Shanshu by helping THEM, not the good guys, win the last battle.
W&H didn't make the prophecy, a prophecy isn't made unless you mean they physically wrote it down as a fake out.
Which I dont think is true cuz firstly they would have made it clear he'd need to fight on the side of evil and secondly the shanshu actually exists, it's not just written on that piece of paper.
The shanshu says that he plays a part in the battle, but doesn't day which side. It's implied in the show that even W&H dont know, which is why they try and tun him, more likely to have Angelus on their side.
Was he lying? Maybe. Maybe not. But if Angel wants his humanity badly enough, there might come a time when he would be seduced over to their cause, the same as he was with Twilight.
No, that just shows that you dont know Angels character at all. In the comics Wesley gives Angel a vision, from W&H, of how the shanshu will be received and it's of Angel on the side of Evil, Angel is so distraught at that he contemplates death so it will never happen, but higher power Cordy and Connor save him and reassure him that he'd never let that happen.
That is not the same situation as Twilight at all, Twilight is a lot more complicated then it's surface value of Angel going evil. Firstly he was approached by someone he trusted Whistler, who had been responsible for the biggest turning point in Angels life and turned him to the path he was on. Whistler says that in all possible futures him working with Buffy results in them dying, and offers an alternative plan to work from the inside for the anti-slayer movement and control the damage they can do, while helping Buffy get to her goal.
Of course he should have told Buffy about it first, but I can't fault Angel for taking Whistler at his word after everything they'd been through, and working from the inside to minimise the same they do is a solid plan, when he doesn't know the damage they are going to do and sometime after taking up that mantle we know Twilight started to change him, cloud his judgement (not my opinion, or excuse - fact)
Saying Angel could be seduced for his humanity is so far from what the character is about, his mantra with the Shanshu is just to keep doing good, he has no idea how much good he has to do, it's not a math equation, but he knows he has to keep doing good. He even signs away the Shanshu (as far as he thought), and even his logic behind the early stages if Twilight are still for doing good.
Thats why I think he puts importance on the shanshu when there were options like the Morah demon. He started out doing good for pure redemption, no reward, but being given a purpose and something to work towards makes his mission easier for him to deal with and process.
Aside from that, there is huge difference between being made human by the morah demon and the Shanshu, firstly cuz he'll have earned it, secondly cuz it takes place after THE apocalypse. If angel had stayed human, there is still a huge amount of evil out there, Buffy would be in danger, he couldn't help in the same way. Whereas if they averted THE apocalypse, presumably evil would take a major hit, things would be safer, there's more of a chance he could settle down with Buffy, or if THE apocalypse took place in 200 years, Buffy would be dead and it wouldn't matter and he'd still want it becasue becoming human wasn't about Buffy, it's something he wants for himself, but something he wants to earn for himself.