It’s not, no widget to drop on = no call. You can evoke drop cancelled, though. I’m actually unsure what is the purpose of Drop in the DragOperation. 
The Dragged is super-useful for what you need - getting cursor position and checking what’s underneath; deproject screen to world and trace.
I can’t remember precisely, but you will most likely need to provide the drag & drop operation a reference to the player controller as these two normally can’t see one another. This screenspace / world space interaction is not something UMG was designed for, it seems
There is another hackaround (apart from your crafty (!) 2 frame delay in order to read cursor data from the controller).
You can cover the entire screen with a visible canvas during drag - that canvas will know what is being dragged over it and where it’s heading:
As you can see, it’s possible to drive both a widget and a world object during the same tick like this. It’s hacky but it does work. From here you can treat that cube as a collider and have it interact with the world.