We saw her becoming a demon again immediately after the opportunity for that presented itself. She whined about missing her powers, the way she could eviscerate people and so on. She was not detached at all. She didn't feel guilt or indifference, she felt pride.
Because the plot said so. Very little of Anya's character makes sense, she is a mess of contradictions because she is barely even a character, she is more of a walking talking comic relief. But if I have to explain it away, I would say it's because she was proud of the creative ways in which she punished people. Doing it with a gun or a knife is just not Anya's style. She is an artist, as far as murder goes. And she enjoyed tricking the women who made those wishes.
It is, but in the case the premise is completely unbelievable. Anya should have known the world of humans far better than say Spike. She has been manipulating them for 1100 years. You can't manipulate people if you don't understand their world at all, the way Anya of S4-7 didn't. Anya of The Wish was cunning, blended among other people easily, was also a good liar and played Cordelia like a fiddle. By The Prom she is retconned into "the funny foreigner".
Also, just like Spike's story, Anya's happened at the expense of the Scoobies because allowing her into their midst was wildly OOC for all of them.