Addressing his ability, it is possible that having things stuck to his face is independent of any fruit ability if he possesses one. It was stated that he was an extremely messy eater which partially explains why there are food items and utensils stuck to his face (though the burger is really pushing the laws of nature... well, then again...). You did fail to address the ability he displayed in his fights against Law and Sanji. He has the ability to harden a stick of bamboo he is holding, creating a shiny black luster on it and his own arm. This may be his currently unnamed fruit ability and it that is the case, it is unrelated to having things stuck to his face.
When fighting against Sanji and the Marines, Vergo used his ability to harden his own body and sustain minimal damage. When he uses the ability, whatever part of his body he wants to harden turns black.
This ability is not a mere Tekkai which would explain his body strengthening. Tekkai does not change the color of the user.
It is possible that the ability is Bosushoku: Koka; a form of haki that hardens body parts and turns the affected body parts black. There is a possibility that this form of haki may be extended to held items and weapons but it has not been described to do so yet. Being a vice-admiral, it is likely that he possesses this haki and if so, Sanji is going to be in for a tough fight.
As for the change of heart, it was also explained that Vergo was a pirate under Doflamingo long before he became a marine. He joined under Doflamingo's orders and was used as his puppet within the World Government.
... But those are just my thoughts :p