Do enchantments or potion effects that 'Fortify <Magic School>' also affect charge cost of using staves?
Solution 1:
Yes. Most easily seen by enchanting yourself 4x -25% destruction pieces (Helmet/Necklace/Armor/Ring).
If you want to confirm quickly, you can use the console commandplayerenchantobject <object> <effect 1> <effect 2>
This creates a given item with the given enchantments withe magnitude of your current enchanting skill. So for example - to get a necklace with fortify destruction:
playerenchantobject 877f1 7a0f6
Combine with modav enchanting <x> (or setav) to make your enchanting 100 to get a -100% cost item with that command.