Ammeter discharge with key on

Frankly I'd bet you are imagining a problem you don't have. You have a meter with an ammeter function, that can measure 20A? Hook that up in place of your test lamp and see what it draws.

Then make sure the interior lamps and all other switches are off, and make sure the brake lights aren't stuck on. Switch on the key, pull loose the regulator plug, and unhook power to the ignition system, and I'd bet your "troubles" go away.

You have any other "maybe" loads like an aftermarket stereo?

While you are doing this would be a good time to see if you have any other unwanted loads with the key OFF. Leaky diodes in the alternator is one possibility, as well as components in a "wired hot" stereo/ amp, and corroded components in things like the interior lighting (door switches)