Basic electricity includes both series and parallel circuits but you really don't need to know everything about the differences unless you are designing a circuit of your own. The model A wiring harness and system is about the most simple set up a person can learn on with exception to say an 8N Ford tractor that even has less circuits. It is simple in the fact that the came with very few electrical components. This is a good link for diagrams.
https://www.mafca.com/downloads/Semi...ex%20Janke.pdf
The lighting system is the most complicated part since it all has to go through that big switch on the front side of the steering box which is kind of a bad place to put a switch but Ford kept it there till 1939. The head lamps have high and low beam circuits. There is a tail light circuit and a stop light circuit plus a park or cowl light circuit. Of all these, the brake light circuit is the most problematic for unknown current draw due to the funky switch that they had for 30 & 31. I'd start with that switch to find the current draw.
It's doubtful that the ignition system would be hot unless the ignition switch is defective. The ElectroLoc switches have been know to have problems after so many years in service.
The horn is not a likely problem area unless the wire inside the horn rod has bad insulation on it.
Most of the later model As have a vacuum wiper if it has a wiper at all and the instrument panel light isn't usually problematic due to it's simplicity but you never know. Most model As didn't have a dome light but they usually aren't problematic unless a rodent got to a wire somewhere.
This brings us back to the big diode in the cut out can for the generator. When you mentioned that it had more draw after it was replaced, that shouldn't be. I don't know what's up with that. I know that poor quality diodes can leak current through but I don't know what diode you have in there. Personally, I prefer the old electromagnetic cut out but a diode should work if it's a good one. It can be checked with a multi-meter on Ohms setting. It will have continuity one way but should have none the other way.