Are copied spells cast from the graveyard?
Your instincts are correct. Copies of spells are just placed on the stack, they are never cast.


  706.10. To copy a spell or activated ability means to put a copy of it on to the stack; a copy of a spell isn’t cast and a copy of an activated ability isn’t activated.


Cards on the other hand, can be copied and cast. For example, if Increasing Vengeance was enchanted by Spellweaver Volute, the copy of the card would be cast from a graveyard.