1. The backwards wiring is mainly for the solar on the side applications, easily resolved/identified with a inexpensive multimeter/volt meter.
2. Normally you want the controller as close as reasonable to your battery for best results. Really depends on your wire gage though.
3. Should be easy to find at the battery for verification but it likely 10 or 12 gauge.
4. Cant help here as I'm not familiar with your model but typically they are MC4 connections.
2020 GMC Denali 4X4 2500HD Duramax
2018 Work & Play 25WAB