...he is, though? Doors have been painted. The people in charge of painting the doors are suspect. And two of them know it wasn't them (albeit they don't know it wasn't each other).
Also, the dwarves notably didn't have any paint, it's probably easy to tell that the paint is dry and was not put on just beforehand, and the dwarves have no spells to magically make the doors appear painted.
Like, sure, you could complicate it a lot if you wanted to. You could also simplify it a lot if you wanted to. So lengthy hypothetical conversations in your favor do little to sway me.