I too am pretty useless at electrics so I recommend K.I.S.S. (Keep It Simple Stupid)
You need to go back to basics because you must have a problem or a short somewhere, disconnect everything out of the system that is not essential. Disconnect the alternator; check the diodes, (if it is an alternator there has to be diodes somewhere on, in or near it).
Check the ammeter against a known load, the headlight bulb will do, just the battery, bulb and two wires. You never know and at the moment you don’t know so check EVERYTHING one bit at a time!
With some spare wire duplicate one at a time the wires in the harness and check each connection with a wiring diagram. It might be wise to check the bona fides of the wiring diagram too, it’s not unknown for them to be wrong! The Scott will finish up looking like a cat’s cradle but sooner or later the readings will make sense. Replace one bulb at a time and measure the voltage across each as well as the current being drawn. Sooner or later you should get big amps and low volts.
The “Ah” rating of a battery is customarily based on a 10-hour discharge rate and is observed more in spirit than anything else so a 13Ah battery should supply 1.3A for ten hours or 13A for one hour.
Jan is correct, you will loose .9V in a 10ft run of 1.5mm wire, that is 15% of the available voltage and with bulbs it is the last few percent that makes the difference between white and yellow.
A short aside about stuff flowing: If you chop a water pipe in half water comes out, a gas pipe it’s gas, an oil pipe, a petrol pipe . . . you get the picture, but if you chop a wire in half nuffink comes out therefore it’s gotta be some sort of black magic! Try this one on any electronic buffs you know, it will elicit a Pavlovian response that compels them to try to explain electricity, cruel but fun if you can keep a straight face while acting dumb and I still don’t really believe in the stuff!