Electric issues

Don't worry about code 12. That just means the battery was recently disconnected.

I don't understand what was done to the alternator. It is not practical to add diodes to it. Also, an alternator is physically incapable of developing more output current than it is designed for, so if something is overheating inside it, the unit should be replaced.

The voltage regulator is built into the Engine Computer. An older, external regulator from a car from the '70s will run this alternator just fine, but doing so will cause the Check Engine light to turn on. The Engine Computer will detect the internal voltage regulator is not doing its job. With the Check Engine light on all the time, you'll never know if another problem develops.

I would replace the alternator, then have a professional charging system test performed. You can start the testing yourself with an inexpensive digital voltmeter. Measure the battery's voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is, that means it's okay to have the rest of the tests performed, but you need a professional load tester for that. What we need to know is "full-load output current", "ripple voltage", and "charging voltage".