PS also, is it actually possible to model a level in Unity and export out for the game at all? (guessing probably not, but curious)
Viewing post in custom levels by users - any sites/place to find these?
It's not possible to do multiple object selections unfortunately. Best you can do is use the keyboard shortcut Ctrl+D to duplicate each object individually. (There is a list of keyboard shortcuts in the user manual).
And no, for technical reasons it's not possible to model a level in Unity and export it.
Hope you are still able to enjoy the level editor although it has some limitations.