Questions:
1. Is Fate in Violentale, Monarch Sans? Or a different person?
2. Why did Sans attack Undyne's dad? Was he hungry?
3. Can't Griz- live on magical Monster Food?
Yep.
Fate is in Violentale he plays a key role as its main antagonist regardless of Route. Monarch Sans is just the key force behind the warp in the time line and such that results in Violentales creation. He visits it from time to time but not often he feels strange about visiting there but he does not know why. He feels guilt but has not learned enough to understand fully what the feeling is!
Undynes father was the one who was ordered to stop Gaster by any means by Fate. Gaster was branded a traitor because of his actions, in order to be certain his research would not be used against his mate he destroyed the information in the hotland facility. Undynes father was in essence the one who murdered Gaster his brother. It was simply revenge in his eyes and injustice that he and the many guardsmen go unpunished for the wrongs they commit orders or otherwise.
A froggit tried to give her monster candy in the beginning it did very little for her. Monster food is typically food that has some magic added into the mix so that monsters can consume it for their energy and magic stocks. However the low concentration does nothing for her body which can't produce magic the same way as a normal monster because of the mudded human part that's become unnaturally merged to the monster half. Raw magic however gives her the energy she needs to sustain her form and self. Monsters in essence are entirely made of magic making them an unfortunate food source. That said eventually in the Pacifist Route Alphys does help create a substitute for this that is in a way like a faux substitute.
Thank you for the questions! I'm happy to answer anymore I honestly really enjoy it.