As far as I know, we can’t currently select every instance of an object in the scene editor. A feature like that would be helpful in cases like when we want to change the Z-Order of every instance of an object, or something similar. Its a pain to manually select each object considering there can be hundreds of instances in a single scene. So a button to select every instance will solve the problem.
I also don’t know if its just me but it seems like we also can’t select every object in a scene using Ctrl+A.