Hi and
to the forum.
If your phone works with the GM App and connects to the Bluetooth dongle you should be able to adjust all settings from the phone and also use the phone as a speedometer, odometer, voltmeter and wattmeter etc. so you shouldn't need a USB programming cable.
If your battery pack is able to provide up to 25~30 Amps, then it should be OK with a single Magic Pie.
The controller does not reduce battery voltage for the lights and horn. The Light and horn switches are powered directly from the main battery, so they will switch the full battery voltage to a suitable horn and/or LED light.
The switches and horn/lighting wires are probably OK for switching up to 2 Amps of current.
You would need to use a suitable 48V to 12~13.8V DC/DC converter powered directly from the battery if you want to run a 12V lighting/horn system.
The required
wire gauge will obviously depend upon the maximum current and voltage requirements of the specific lights and horn that you decide to use.
Alan