Will a "fortify destruction" potion increase the strength of an fire-enchanted weapon? And or, will a "fortify illusion" potion incrase the level cap of the fear-enchanted weapons? Will these "fortify" potions work for Staves?

anybody?

Not sure, to be honest. I'd doubt it though - weapons are governed by the one handed and two handed skills, and enchantments are governed by the enchanting skill. It'd be very unlikely that your destruction skill would affect enchantments.

Fortify destruction will reduce the costs of that school, including enchantments, so you will get more uses out of the weapon enchantment.

The fortify destruction potion will increase the damage of the destruction enchantment, as well as the spells.

Fortify illusion enchantment will result in you getting more uses out of the enchanted weapon, but not the strength, thats affected by perks though.

The fortify illusion potion will increase the effectiveness of illusion enchantments.

You're sure? I know the "fortify" potions increase the strength of staves but even weapons? How do you know this? I tried experimenting with it but I couldn't see a difference

Will fortify illusion also increase the power of a poison of fear if I place it in someones pocket (poisoner perk)?

I think someone needs to test this out with fortify illusion potions, I never use illusion poisons or enchantments as they're too weak for normal use. It works with destruction pots (enchantment damage) so I assumed it would with illusion pots, I doubt potions will affect poisons.

Ok i'll try to test it myself. I'm trying to make a decent Thief build that can work on Master difficulty, using powerful poisions and potions to destroy enemies. If i'm out of potions, I'll just steal their stuff and sneak away...