As for atronachs:
Flame: Deals most raw damage. Exept against those fire resistant or fire immune.
Frost: Best tank, since it is immune to frost.
Storm: Only itself is immune to the shock-type.
I find it best to use a Storm Atronach and Dremora Lord.
Wards block any damage, but are particularly useful for dragon breath. Stones, don't use Atronach with conjuration (unless on PC with unofficial fix), Lord is good, as though the armour bonus is small (equivalent to 6% shield spell), your Breton has 50% magic resistance without enchants (and we all need as much magicka cost reduction as possible with multiple schools) or Alteration perks.
I have a Breton mage/archer. Using the Lord birthsign... I haven't decided yet whether it's a game-breaker or not... but you are pretty powerful in terms of static defense. Will probably trek east someday when she has business at the college and swing by the Ritual Stone and re-acquire that... Nothing sez awesome more than instantly having up to 6 followers to start the beatdown or turn the tide.
A Frenzy Rune would be awesome for illusion/destructo types.
http://www.uesp.net/wiki/Skyrim:Ward
25% racial and 25% for Lord stone.
The Atronach stone is the best for a Breton because the racial power stacks with the stone power giving you 100% absorption for 60 sec. This makes you immune to Dragon Priests. Plus, you have 50% absorption and 25% resistance just walking around normally, not to mention an extra 50 magicka.
The downside of slower regeneration is very minor if you have regen fortify items or cost reduction.
If you're on xbox, they will fail 50% but you don't lose magicka so it's just an inconvenience (at least that's how it works on vanilla PC). If you're on PC and run the unofficial patch, it's fixed.
I usually take the Mage stone for my mages up untill level 20 - 25, then switch to Lord.
Illusion is a great powerful skill for any character type. Only thing is to get the most out of it, you're pretty much gonna have to max out all the perks.
Some spells require a little patience. Fury can be especially annoying to use at first in a combat situation, while calm is excellent. Fear trumps them all in near-death situations as well.
Yes. I use it in combat often. I like distractions, and illusion provides just that.
It takes a little practice, but if the AI are in frenzy and closer to each other than you, they'll start fighting, and you can laugh maniacally... or handle another task. I prefer the former, though.
'None that come to mind. Just remember to be diverse with magic. That'll make it more powerful than any other weapon in the game.