LTC4020 Design - 4S lithium battery

I'm designing a device which will be employed in cars and trucks, so it must withstand 12V o 30V as an input (battery of the vehicle). Additionally, 4s Li-ion (16.2V) will be placed in our devices as a back-up battery since if he vehicle is turn off, we need up to 30min of monitoring to see if something happens inside the car. I saw LTC4020 and it fulfills all my design requirements; however, I have some concerns about its design:

- Datasheet explains DC/DC converter operation and its Vout goes to Vbattery and Vsys. Does the Operation Mode depends only of Vbattery? For example: Vin = 12V, Vsyst needs 12V and Vbattery is 16.8V (4s Li-Ion fully charged), DC-DC converter will enter in step-up mode to charge the battery. What happens, when there is not battery connect to it?

- Does it need a Back-to-Back Fet to protect the battery?

- Will it desacouple the battery to the system when there is an external source? Otherwise, it will automatically connect to the battery.

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