I haven't seen any reference, pro or con, to using fatigue to enhance cannon attacks...maybe I have simply missed it. Is that allowed? How about adding power potions to cannon attacks? Also, is the Overlord allowed to spend threat to add power dice to cannon attacks? Since skills and hero abilitiies may be added to cannon attacks, I suppose it follows that fatigue can be used as well. On the other hand, thematically, what exactly can a hero (or monster) add to a cannon's effectiveness with "extra effort"...? Thoughts would be much appreciated. Thanks in advance!

When you use a cannon, it simply replaces the weapon you have. So, you could use fatigue or even a power potion.

Yes fatigue is most definitely allowed with cannon attacks as are power potions. Although I suppose you could claim since monsters abilities cannot be used with cannons that therefore threat could not be spent on cannon attacks, I would think that threat could be spent. It isn't really a monster ability, it is simply the overlord upgrading or adding to the power dice of an attack just like any other attack.