1. You need to remove your wire shown from battery to Main Fuse A. Then wire C feeds Main Fuse. The way you have it labeled (Main Fuse) , flow for Main Fuse will be in (wire C) at B and then out at A.
2. Is this an Alternator warning light? I am not sure, but think if it is wired so it lights when current flows from key switch towards the alternator it should work. LEDs have a certain direction for current flow that they will light, other way they will not light.
3. Both wire A & B, to Fuse Box should come off A of Main fuse. Flow is Battery to starter. Starter to Main Fuse B (wire C). Though Main Fuse and out of Main Fuse at A.