Im currently running a lv 18 Breton Illusion/Conjuration/Restoration (soon vampire) mage. I am running 100 magicka because I plan to get 100% cost reduction in said schools of magic.

Last night I abused the infinite magicka for 2 hrs college of winterhold focal point glitch to get lv 100 illusion, and get all but the half magicka spell perks, including master of the mind.

Some questions:
1. Advice on stat ups? Should I go for about 200 stamina, then dump it into health, 3:2 ratio of health to stamina ups, or, with so much protection with illusion, followers, wards, and Conjuration, do 100 hp all stamina?

2. Should I invest 2 perks into sneak for muffled movement, and get lv 100?

200 Stamina would be good. You say you plan to throw everything into Health, but I still think you should put in some Magicka.

Stamina, to me, as a mage is a waste. I'd get your health to about 200-300 and put the rest into magika. Even with the cost reduction, those high level spells still cost a bit.

Also note thatIm on legendary

Even though you're going to do cost reduction for magic, the thing is you're a mage, first and foremost. Your magika is your lifeline and therefore should be your #1 priority when leveling up.