Hey Vargr! BB her with a little something about Shock Magic!

After running an outright war against the undead with my own undead "Seige-mage" (I won't get into details), I found it was easier to hunt undead with shock instead of fire! My theory as to why:

Shock has Disintegrate, which "reduces" the max health of the opponent. Since that's the case, throwing Necromage into the mix of Augmented Shock and Disintegrate increases the Damage of shock spells and makes it easier to reach that 15% for Disintegrate to trigger. With Chain Lightning, some Alchemy and all the perks and two Dual-casts, I was able to fry four Draugur Death Overlords before they could even touch me.

Question: Does the taper damage stack? If an enemy is lit on fire twice in a second, will the taper damage stack, just dealing more damage where the two burns overlap, or will one cancel out the other? If that's so, thunder has an even greater advantage because the 15% boost on thunder is fixed, and doesn't depend on time.

Do these effects also benefit enchantments?

The taper damage does indeed stack.  Which is why burst casting flames is more effective than a steady stream, and why Ignite is such a powerful spell.

Augment perks increase the amount of elemental damage enchanted weapons deal when you are the wielder, but not the base amount of the elemental damage when you are the enchanter, only the Elemental Enchanter perks do that. 

I don't believe the Intense Flames, Deep Freeze, or Disintegrate perks affect enchanted weapons, at least I've never seen them trigger with enchanted weapons.

Fire spells' taper mechanics (scroll up a bit, I posted a link above) seem to suggest that Necromage fire is an incredible undead killer, though.

On the other hand, Chain Lightning is indeed the best spell in the game (subjective statement).

I won't make any statements about tapering for flaming weapons, but I believe that the elemental effect perks in Destruction (Intense Flames/Deep Freeze/Disintegrate) don't affect enchanted equipment - so a nick on a 14% hp Draugr Death Overlord with a 5 Shock Damage Iron Dagger won't trigger disintegrate. Hope that helped.

Yes, but Waiting for the taper vs instant death. That combined with near instant travel speed.

I thought of a disadvantage of Desintegrate: wouldn't all those ash piles bloat the game data (though I'm not sure if that's as bad as it sounds)?

May I suggest using the Iron Kingdoms setting from Privateer Press for reference?  The Warcasters of Cygnar are known for using electrical attacks.