Chieftess,
You pose an interesting question. I am sure the FP/palace rank exploit expedited the victory.
I am not so sure it would affect the outcome however. It is absolutely possible that, at any time before China fell, the AI had the military power to squash Japan like a bug. Not using that exploit would have extended the time when Japan was in that precarious position, and it would have extended the amount of time it took to reach the conquest victory, but I am skeptical it would have affected the outcome. The exploit was first put into effect in 1250 AD, after some of the worst danger had passed. The game ended in 1846, leaving over 150 turns left to overcome the increased corruption from not using this exploit. I can assure you that the original FP placement was not done with the rank exploit in mind it was done to help the furthest cities and save a few gpt by selling nearby courthouses.
Your observation was from loading up the last turn of the game into C3C, right? At that time I had the palace placed far away from any good cities. If I had been using the forbidden palace as it was intended, then both the palace and the FP would have been surrounded by fully developed cities. I am sure proper palace and FP placement in C3C could not have matched the low corruption level I received in PTW, but it could lower it from the -479 you saw. In effect the exploit in PTW has disastrous results in C3C.
In addition, I was way too conservative in conquest of the second continent. I was building units faster than I could use them up. If I had it to do over, I would be much more aggressive, and therefore affect the outcome because of what I learned the first time (spoiled as you mentioned).
For now I will pass on this challenge. I have already spent 142 hours or so playing this one game (plus how many hours documenting?), and I do not think I want to do it again.