Try disconnecting your alternator completely. It may have a big short in it.
If not, disconnect everything wherever you can and measure the battery voltage. Then add things slowly until you get a drain.
The alternator is new. So far, I've tried the stock alternator, a newer alt with internal voltage regulator (did the bypass trick) and now a new alt with external regulator hooked up to a brand new voltage regulator. All produced same results. I'll try to run it without an alt altogether and see what readings I'll get! Haven't tried that yet!
What I'm seeing after a quick look at the schematic for a Z (I'm new to these but work on electronics for a living) is a couple things.
First, the indication that turning on the lights works but your voltage goes to 0. That indicates that wherever your voltmeter is wired to is seeing ground. I need to know where that tap is.
Second issue is that voltage spike while running. I'm betting that wherever the sense wire goes is shorted to ground. That would cause it to push the alternator high.
A few possibilities from looking at the short issue for a few minutes: bad light switch, bad tail lamp (if removing the bulb fixes the issue that's it), bad dimmer.
I'm going to bet it's two seperate problems.
I get the same symptoms with pretty much every accessory, except for turn signals. At times of short, turning off the car and removing my key brings voltage back, but simply inserting my key into the ignition (without waiting a bit) results in the drop again, even with everything off.