Fortify Destruction and Magicka Regen enchantment on armor

Solution 1:

I'm having a little trouble determining exactly what your problem is, but I will hit your list:

1. I believe without exploiting potions and recursion stat boosting, the maximum percentage of magic reduction on a single enchantment is 25%. This is with 100 Enchant, 5/5 enchant stat, and the various corresponding element/weapon perks.
2. Magnitude is not affected by armor type.
3. You should be able to smith and enchant Dragon Scale Armor with 100% Destruction magicka reduction(free cast). Additionally, with the extra enchant perk, you could boost other school's cast regen, or eliminate it completely.

Remember you get the maximum enchantment power when using Grand or Black soul gems.

Solution 2:

Base value for enchanting destruction is 8 points so u can get 25% reduction with all the perks, but the destruction base value for destruction & mag. regen is 5 points and magicka does not change, so it will always be 10%, that's why u get only 17% reduction. You can get higher % reduction using potions to fortify enchanting but magicka regen will continue to be 10%. The only way to change that is using mods.