a pure mage without destruction or any combat skills to play on either master or expert, is it possible to do?
Absolutely! It takes a little bit of doing-- and restraint, but the bottom line is still the same. (Honestly, playing on Master or Expert would be quite drastically easier as well...)
So your ideas and input would be so appreciated.
As mentioned above: Staves are incredibly useful/vital/mandatory... But, you already knew that. Use a staff instead of the spell, Destruction stays the same. Excellent way to stay pure and be less than defenseless.
Alteration and Restoration should be diligently indulged upon. Running away, healing, taking advantage of flesh spells-- and PARALYZE! All of these will help incredibly in turning the aggro on whomever you run with, or to your conjurations. Which brings up Conjuration...
There have already been some good tips on Conjuration, but one I haven't seen to date is: That you can turn around and cast on your follower three times before they will attack. Of course, they'll have to like you first (or else they leave your service instantly); but, regardless, it's a wonderful way to level it
"on the fly"... ...Just make sure you wait some seconds before restarting the process. Casting it a fourth time will make them hostile, leave your service, and make it inconvenient to win their trust again.
I haven't made my mind up about stone blessing yet, so haven't bothered as yet.
The Atronach is great for leveling Alteration and Conjuration as well. Conjuring a Familiar will 50% of the time (though, not actually 50% of the time) be absorbed back into your magicka pool. Cast some Familiars, absorb some mana, cast a flesh spell; to infinite...
...and if you're looking to grind your socks off, do this near an enemy that cannot reach you, and you'll be set for life.
The last bit I can add to Illusion is... Well, nothing... Unless, as already said, you keep up on perks and buying the next tier spells when available. Illusion loses its effectiveness
very fast if not kept up-to-date. That said, it
is great for crowd control when you have sufficient school reductions, mana pool, and meet the level requirement. It is a very fun school, but I would never use it as a primary school of magic.