Battery fried and more

Vreg controls the voltage it senses, which is not the battery voltage, but rather a point after the key switch (usually IGN1 wire). If there is a drop to that point, it will try to make that point the desired 14.3 V (rel. to its case), which could jam the alternator at full output, trying to make Vreg happy, which could be 18 V at the alt out stud. Use a multimeter and wiring diagram to measure the voltage (from BATT-) at these critical points. Excessive drops thru corroded firewall terminals are very common, and there are many, many posts on this so search if what I detailed is confusing.