None! The Bolt is an OBD device in that it simply easily plugs directly into the OBDII port - this is a zero-install option and is convenient. It only draws power from the OBDII port.      
There are 14 pins on an OBDII adapter, but only 2 (Vin and GND) are connected to the actual PCB circuitry on the Bolt2.

The Bolt2 detects trips based on movement (emulated ignition)     
See Wired Devices - Overview of Operation

The Bolt2 has an internal battery fitted. In TG - configure an "External Power Removed" alert using the wizard. See External Power Removed Alerts - Bolt2

The Bolt2 can easily be installed with an OBD adapter cable or a Y-splitter cable - readily available from many providers. See here - Bolt 2- Assembly and Installation Guide - when installed in this manner - you won't even know it's there. With the Y- splitter, mechanics will still have access to the OBDII port for diagnostics, and won't pull it out and forget to plug it back in when servicing. With the adapter cable, mechanics will have pull out the cable to perform diagnostics but with the internal battery connected, the device will not cease connection. 

Yes, see Harsh Event Setup - Bolt2 and Accident Detection and Reconstruction + Accident Data Exports - Bolt2  
For speeding alerts, the device will report it's speed, and then the end platform can alert on this.

We use high quality components and make use of GPS Aiding Data to ensure the best performance. Typically, speed is accurate to within +/- 2km/h. But GPS isn't perfect or 100% error free, it can in rare instances be wrong, see here: GPS Settings and GPS Accuracy.

No, the Bolt2 is only available in cellular NB-IoT / LTE-M.

Some manufacturers will claim this, it is simply not likely. Can the device flatten a vehicle battery? - Bolt2

Two reasons for a Bolt2 to record 0V for the internal battery reading:

Internal Battery Plugged in:

If the internal battery drops below 3.35V it will record 0V.

Internal Battery Unplugged:

When the devices exceed in 40 deg C (or below 0 deg C), the charger is disconnected to preserve the battery health. Usually with the internal battery connected, the internal battery reading would read 100 mV lower after the charger is disabled (4200mV -> 4100 mV) . If the internal battery was disconnected and the device is operating as usual, in acceptable conditions, the charger would be powering the battery terminals and thus the internal battery readings would read 4100mV. Once the device is outside the safe temperature zone, the charger stops powering the internal battery terminals and thus the datalogs will read 0V.

For information detailing how long your backup battery will last, navigate to: How long will my back-up battery last? - [Bolt2]