Very Interesting.... couple of questions.
First, with the switch in headlight position, the current at the battery minus terminal is 10+ amps, right?
Is this the reading you get when the headlight has blown?, Or, do you get this reading with the headlight on and shining?
Reason I ask, if the reading is 10a with a blown headlight, that may be o.k. You've just switched on extra generator output, but it has no place to go except the battery if the headlight is blown.
Next, have you blown a taillight while doing these tests?
IF not, then here's one way to look at the problem.
You say the voltage is o.k, and because the taillight hasn't blown, that seems to reinforce the theory that the voltage IS o.k.
Which leads to one conclusion, something's wrong with the headlight and or bulbs.
The usual blowing headlight bulb problem is a bad ground between the headlight and the battery. But, they usually don't go "POP" like flashbulbs if that's the case. It takes awhile.
I doubt that it's bad bulbs, 'cause you've tried two of different types. But it could be.
Try this if you can;
Get a different voltmeter, to verify your voltage readings. Either a high quality digital or a good quality analog.
IF the voltage is really o.k, then make up a test jig with a headlight bulb so that you can connect it directly at the battery with alligator clips, or better yet, test leads screwed to the terminals. See if the lamp will survive. If it does, great, we've verified the generator and battery.
If your test rig has alligator clip leads, move to the ignition switch, and hook to the headlight terminal and a good ground on the frame near the switch.
If it still works o.k, we've pretty much narrowed it down to some sort of wiring problem to the headlight, so go after it.
This is all based on the premise that the voltage IS GOOD, and the taillight has survived to date.
Keep up posted on what you find.