Can conjuration get 100% magicka reduction cost like destruction? I mean casting conjuration for no mana?

Yes, with enchanting you can make two schools magicka-free. (Two per gear set.)

not that I've seen and My skill is 100 with all conjuration spells.

Yes it can, the person in the college that sells conjuration spells also has robes that you can buy to disenchant and break the game... Conjuration wont really break it though.

Yes, although full 100 isn't even needed, since you'll still regen faster than you could ever practically raise, or summon.

So that means I can keep casting dremora lords if one dies????????