Getting various non-zero voltages between neutral and ground on daisy-chained extension cords
Keep in mind that voltage drop in the wire is totally load dependent; when you measure voltage without load at the end of long wires, you won't see much if any voltage drop.  If you use a splitter on the end of your chain of extension cords, and plug in your shop vac and turn it on, and test voltage on another receptacle on the splitter, you'll see voltage drop.
The small voltages you're measuring from neutral to ground could be induced voltages, and will disappear if any load is placed in the test circuit, or if you use a low impedance volt meter, which is in itself enough of a load to make the phantom voltage vanish.