Your are correct that the backside cavity is not visible. You can see it by clicking CTRL+D on your keyboard or clicking on 'View' then 'Outline mode'.
For the global change to inches please make sure you select inches in the Units and inches in the 'Grid distance' in 'Settings'. Also select 'set as default' and click ok. Please see the attached screenshot of how it should be setup.
Please don't hesitate to contact us if something is not clear or if you have more questions.