More strange electrical issues

What started out as an innocent question about an extra fuse box relay has morphed into more questions. I now understand that my (Euro 06236) car is pre-wired for A/C despite not having it installed, and that the fuse panel reflects this pre-wiring. The car has an extra fan relay that can be activated by the A/C system, as well as other wiring that now makes sense given my original fuse labels. The labels are different from the current repro labels. And of course the wiring diagrams I've been able to find reflect none of these changes.

My latest questions arise from my car having a non-stock Bosch alternator, I think from a 308. It has a built-in regulator (I still have a correct Euro-style black regulator on the fuse panel, but it's not connected to anything). The alternator exciter wire (white wire in second from left fuse position below) is connected through the fuse directly to the accessory terminal of the ignition switch, which supplies 12 volts when the ignition key is in position 1. The left two fuse positions both get the same accessory-position power. The left-most orange wire is for A/C and indeed as you can see below my original label says "condizionatore" in that position, although the English translation "heater" is a little misleading.

The wiring at the 5th-from-left "regulator" position is actually vent fans (blue wire) and stop light (red wire). So going by my original label, the exciter wire and the vent/stop wire positions are swapped and everything else is correct. And normally the exciter wire would be connected to the external regulator output, and its red B+ wire would be connected to the 12 volts on the fuse panel.

I innocently thought what can be the harm in swapping them back? Then everything matches my original label. Great idea, let's do it! I then confirmed the alternator exciter wire was getting 12 volts with the ignition key in position 1, and the brake lights and vent fans were getting 12 volts in position 2 (the middle fuse positions all get power in position 2). I started the car, and everything seemed happy. Alternator is charging, and my brake lights and vent fans work. I then turned the ignition switch to position 0 to stop the car and things got weird. The tach needle started bouncing around and the car kept running. Not dieseling, just running normally.

I disconnected the ignition at the Marelli box and killed the motor. I then swapped the wires back, started the car again, and was able to stop it normally.

So what the hell? I've thought about it a lot and can't understand what is going on. How can the alternator exciter wire affect the power to the ignition box when the key has switched that power off? Why does the exciter wire affect the ignition box when wired to the ignition terminal, but not when wired to the accessory terminal? And why does the original regulator setup not suffer from this weird behavior?

I'm very confused at the moment, so if anyone can explain what's going on, I'm all ears.
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Ugh - what I meant was after the ill fated swap, the alternator exciter wire was getting 12 volts with the key in position 2, and the brakes and fans were getting 12 volts in position 1, just like you'd expect. Sorry for the confusion.

 

Only US cars had the red regulator under the dash. Euro cars had the black regulator in the fuse panel.