I would not want Ctrl to be used for the copy entity function because the Ctrl key is used to orbit vertically around the mouse click point.
When clicking the left mouse button while pressing Alt and moving the mouse left and right: the camera view orbits around the click point horizontally and when moving the mouse forward and backward, the camera view zooms in and out.
When clicking the left mouse button while pressing Alt and then holding the Ctrl key down too, the camera view then orbits vertically on the click point when moving the mouse forward and backward instead of zooming.
I am using your inspect.js and also your edit.js. I don’t see any action at all when pressing the shift key. I am not sure what you mean about displaying the X and Z axes, maybe it was added after you modified the edit.js?
What ever the reason, I (and many others) would much rather have the shift key used for the copy function rather than using Ctrl and thus breaking the orbit vertical mode.
At one point I heard arguments about how it affects users in HMDs. I don’t think this really applies because there are no Alt, Ctrl or Shift keys on hand controllers and any editing done with hand controllers is inherently different than editing with a mouse and keyboard.