Hello
you will find in the user manual that there are list of certain requirements to be met for the EV mode to work. If one is lost then it switch back to hybrid mode. Now the colder weather is coming you wil find it will start more in hybrid mode, but after a mile or so you will be able to go in to EV mode, But if one of the list is lost driving a long it will go back to hybrid mode.
As to the tail gate you will find that after a time the car goes to sleep and when it does that it times out the tail gate and just opening a door wakes up the car and the tail gate will close.