I propose the following additions/modifications in the rooms editor :
1) When the user press the left button, if an object is present under the mouse cursor, it moves the object, if there is no objects, it adds an new object. If the ctrl key is pressed, it will add objects, and not take into account if there is already an object or not. This is the behavior of GM Studio, and i think it's more logical.
2) Global move of instances. It is present also in GM Studio.
3) A white rectangle around the selected object will be displayed.
4) An help button to display the available commands in the rooms editor.
If the ctrl key is pressed, it will not copy objects, but always add objects.