In Jack Slash's Interlude, he's considering the various levers he uses to control the S9. For Mannequin, it's simple: he's "predictable, manageable" most of the time, but when he steps out of line...
Mannequin had his mission. Few things bothered him as much as seeing someone try to help others and succeed where he had catastrophically failed. To keep Mannequin in line, Jack could remind Mannequin of who he had once been. A simple casual utterance of the name 'Alan' served as effectively as a slap in the face to someone else.
I do think Mannequin can be considered a different person than Sphere, in a certain way. But the fact remains that reminding him of who that person was causes him pain – and his response to pain is to flinch back into himself, becoming more of a machine and therefore easier for Jack to control and direct.
This is exactly the trait that Singularity is mocking him for in the spoiler snip. In Singularity's eyes, Sphere is disgusting because he abandoned all his dreams of helping the world the moment his own life was ruined. And, even to this day, reminding him of his old works causes him pain, which then leads to him – once again – reiterating his choice to become a faceless monster. To hide from the pain.
So Singularity mocks him in the same weak place that Jack does, but with a very different context and motivation behind it.
