Here is my planned build. Dual wield bound swords. Sneaking with daggers. Illusion and conjuration for situations which require ranged.

1) Do I need to take the master illusion perk? Are the master illusion spells necessary / useful?

2) Could I get by with just a frost atronach, thereby removing the expert level conjuration perk?

3) Any other suggestions?

I never have enough perks for the characters I want. I would like to get the recovery perks in restoration, and use destruction too, but ah well.

For #2, you can make staves for Fire, Frost and Storm Atronachs so if that's all you need you don't have to have any Conjuration at all.

I filled the entire Illusion tree. Extremely overpowered. I literally ran through Chillwinds, and paralyzed or frenzied all those Falmer into destroying each other. Add Conjuration and Enchanting to this, and you have a god using only 3 trees. Of course, the challenge will be gone, but to each his own.

Even though I have a level 74 mage, I've restarted a new one just so it isn't so powerful.