Plastic wrap is porous, so oxygen will slowly seep in turning the very top layer brown. If you make the guac the day before, it will be a thin layer and can be mixed into the dip. Don't slice an avocado until you need it. It will go brown in a few hours. It's a chemical reaction between the avocado flesh and oxygen. It's not harmful, just unattractive.

yes, apply the plastic wrap directly to the surface of the guacamole to prevent air from reaching. It's a chemical reaction with oxygen that causes the browning. Acid helps delay the oxidation and enhances flavor, but won't stop it.

yes, but take out 30-40 minutes before serving to warm up a little. Don't remove plastic wrap until ready to serve.

did you see the water trick. pour enough water to cover surface, don't mix though, then drain when ready to serve. probably saw this already but whatever