We'll start from the top.
Your passive I like. I don't like the % though. I agree that it should be something like 5/10/15%. I've seen shaco's that stack crit runes and do 300 damage on a deceive at level 1. Considering most champions have about a 5% crit rate at level 1, 5% would barely be noticeable except for very specific champions. I don't see this as overpowering.
The damage on your Q is laughable. 400 damage on a 12-second cooldown at level 18 isn't anything to run away from. I would definitely up the AP ratio on your Q to .3 or .4.
The damage from your W seems easy enough to avoid as well. If the slow has a short duration, make all the mirrors shatter upon completion, and let the AoE be large enough for all mirrors to hit the center. If that means keeping the AoE inside the circle, that shouldn't be bad. Make the duration longer.. say 4-5 seconds. It's a slow. They can still attack their way out, flash, shunpo, use shields, kill you, etc. Anything shorter will be useless for catching/escaping, and make the damage too powerful. Anything longer would make it too easy to escape.
E seems fair enough.
Me likey ultimate
Mana costs across the board seem high. Your lowest CD spell doesn't do enough damage or cooldown quick enough to reliably farm with or even last hit, and the cost on it is rather high. Hell, Veigar's Q gets a MUCH better AP ratio, has 1/3 the cooldown, costs 75 mana at all levels, and farms AP. Sure you can argue that this is a support champion and not meant for damage, but any champion that can't scare off an opponent at low health is going to get roflstomped. Keeping AP ratios low prevents op cries and nerf batting, but it also promotes physical builds like you see on Janna. We don't need support champions attacking 2.2 times per second dealing 300+ damage.
I like the idea of this champion though. Laning against champs like karthus or eve will keep you juiced up (for what, idk... terribly long cooldowns), while going against tryn or kat and the like will prove much more difficult.
Consider Zilean's spells as a fellow support champion. Also consider that Zilean has more mana than your proposed hero.
Time Bomb - high damage AoE offensive. Semi spam-able with his W.
90 / 145 / 200 / 260 / 320 (+1.1 per ability power)
60 / 75 / 90 / 105 / 120 Mana
10 second cooldown
Rewind - lowers all Zilean's cooldowns by 10 seconds
18/16/14/12/10 second cooldown
Time Warp - Speeds up ally/slows enemy
Chronoshift - Zilean's ultimate. Revives allies upon death.
Revives with 500/750/1000 (+3.1 per ability power)
180/160/140 second cooldown