Based on Mary's statements, what do you infer is the real reason Mary gives Elizabeth the gift?

No doubt Abigail had asked Mary to do this. Abigail, by this point in the story, knows it takes very little to be accused of witchery. Abigail knew that the poppet (doll) would certainly look incriminating in Elizabeth's house. Abigail merely had to go into her theatrics about being stabbed with pins and Elizabeth would be promptly arrested.