Human Necromancer Build Help.

(1) It depends on difficulty settings. If Walk Softly is enabled, random elemental resistances issue an irritating challenge to mages unless you are using Blade of Tidarion with runes. While most enemies are not immune to electricity in vanilla game, the most common resistance seen on promoted ones is electricity. It puts inferno-based abilities into priority by comparison and storm ones as second.

(2) Given the nature of Simulacrum, defense does not matter that much to Necromancer so long as you do not fall unconscious twice during its internal cooldown (20 or 30 seconds? not sure). In fact you can even use Berserk masterwork materials for additional damage boost when not soloing, hence spending half dozen of abilities points into Winter tree to get Chilling Array + Ice Armor is not cost-effective. Wall of Fire's CC utility is not reliable against most humanoid enemies unless paired with Static Cage; additionally the spell cannot critically hit, has relatively low damage (800% over 8 seconds, or 1200% with upgrade) and does not benefit from Chaotic Focus. With the help from Ring of Doubt, casting spells in proximity is not that risky even for ranged characters and two quick succession of Flaming Arrays can easily eliminate whole bunch of normal mobs even in level 27.

(3) It gains some sort of utility in Jaws of Hakkon and Descent, otherwise not worthwhile. Even in those conditions, I guess Confusion Grenade is a more instant way to play dirty tricks against pesky enemies.

Seriously, the contents introduced by Trespasser are heavily biased towards Knight-Enchanters among three mage specializations, in contrast to popular belief. Nevertheless a Necromancer can work a way out whose motto is gonna be "no defense attached".

My build would be:
Spirit (5) - Barrier, Dispel and Rejuvenating Barrier
Storm (5 or 6) - EB, Conductive Current, Static Charge and Lightning Cage
Inferno (8) - All except Wall of Fire and Flashfire
Winter (2) - Mana Surge (you can respec for Fade Step in dragon fights)
Necromancer (7) - All except Spirit Mark and upgrade for Horror
Remaining ability points can be used to upgrade Aegis of the Rift and/or get Strength of Spirits.

Active slots:
1 Barrier (Elegant Defense)
2 Dispel
3 Energy Barrage (Energy Bombardment)
4 Static Cage (Lightning Cage)
5 Immolate (Wildfire)
6 Fire Mine (Flaming Array)
7 Walking Bomb (Virulent)
8 Mark of the Rift / Aegis of the Rift

Use Ring of Doubt, crit-focused gears (Superior Prowler Armor, or Superior Battlemage Armor if you have Avvar Cotton) and heal on hit masterwork.