Note that in Lone Wolf, you get a lot of points, enough to max more than 2 schools. Even assuming you get stuff like crafting/blacksmithing and some random points, it's very easy and highly recommended to have all schools at 1 or 2(depends on the school) to get various early buffs:
Witchcraft: Oath of Desecration, Blind, Absorb the Elements(need lvl 2 to remove AP penalty)
Air: Teleportation, Blitzbolt, Become Air(lvl 2), Invisibility(lvl 2, mostly useful for stealing stuff and doing the sneak stuff without actually sneaking so lvl 1 is fine since no AP is required out of combat)
Earth: Fortify, Midnight Oil, Summon Spider, Bless(for your pets, otherwise useless), Boulder Bash(lvl 2, insane control spells since it's an aoe knockdown on low cooldown that isn't blocked by smoke and such), Earth Shield(lvl 2)
Fire: Wildfire, Flare, Fireball(lvl 2), Summon Fire Elemental(lvl 2, useful against fire mobs and there's a lot)
Water: Minor Heal, Rain, Ice Shard, Slow Current, Ice Elemental(lvl 2, useful against water mobs but not that common).
It's better for both mages to be able to buff themselves than have one buff both, especially since you'll probably want to buff your summons too so they don't get crushed too hard.
Haven't seen a mob immune to all spells yet. There's a lot of mobs immune to specific magic damage types but they're generally weak to something else so you just use that. That's why having all schools is useful for at least having a nuke that works. Otherwise yeah just rely on summons, Spider, Wolf and Bloodswarm do physical+earth damage(like weapons that have earth dmg so mostly phys and a bit of earth) and Skeletons from Witchcraft do pure physical, while the elementals do their own elements so they wouldn't be good if you wanted neutral damage.