JourneyMan puzzled me too. Also in my case the provided screws were too short for the camera in the case. I'm still puzzled regarding the additional angled rubber grommets (like which way they should go), took me an hour to examine first the contents.
I agree, for the price not including a basic mounting diagram is not what I was expecting.
The good news is the camera itself is waterproof, case itself isn't, so you're good to go without any case.
As for side cameras, I used small cable tie around the camera base inside the mirror tightly wound, as I wasn't convinced rubber will hold it.