The Orion XS will reduce the charge current if input voltage is close to engine shutdown. This will keep the charger charging.
This is stated in the manual page 11. Your car is probably euro6 and the recommended thing to do is not to use engine shutdown detection. Instead use the input to turn off the charger when engine is not running.
Yes.
By memory,
H is for activating with +
L is for activating with 0v
So you will need too use only one, but can use both if you eg. Want a manual override for start charging.