Android 5.1.1 ROM Customization for IoT Device

My company is making a hardware device running under Android 5.1.1.
Basically we have one single app that should stay "full screen" all the time and the user should not access the underlying Android System.

As we are entering production we need to finalize the ROM Customization.

1) Debug Mode vs Production Mode :
Prior to delivery :
- close ADB interface with code and switch to USB File Share Mode
- find a way to re-open ADB by using some special "tap" combination on a certain screen.

2) USB File Share is always on and locked (no confirmation asked to the user)

3) Never exit our App and never see Android Icons or Menu

4) Hide every popup message and notifications

5) No power off message (default behaviour is shutdown when long press on the power button)

6) Customized USB File Share
- don't ask any user confirmation
- make the PUP appear as a regular USB storage
- display only the content of some directories we can choose
- fire notification events (intents?) in the system when documents are accessed on the storage

7) Android System Update without asking the user permission

8) Optimize boot time

9) Optimize energy saving on sleep mode