The best solution for your 4th question is to factory reset the DX and set it up under a new Gmail account. That way, they do not have access to your Gmail account and they cannot purchase apps in the Play store unless you set up a payment option for the new account (usually when you make your 1st purchase). Of course the con is they would be restricted to free apps only.
Another option is to root the DX (very easy) and purchase Titanium BackUp
or similar app where you can 'freeze' apps you do not want them to access (Gmail, Play store, etc).
Note for question 3: After your DX is removed from the account, you can turn off the cell radio
to extend the battery life a little. WiFi will still work fine.