I'd just have it automatically decrease any threshold by two - (inversely) like Bless This House, under the same exact circumstances.

I'd thought about that, but it seemed too passive for that kind of character.

Interjecting a possible True Faith power (For those of Evil Bent)

A Pox on Both Your Houses [–1]Description: Your faith in evil is such that even the protection of a threshold withers under your glower.
Skills Affected: Conviction.
Effects:
A Pox on Both Your Houses. By your very presence in or outside of  a place, you may attempt to decrease the strength of its threshold — assuming your abilities aren’t already dampened too low by a threshold for this power to work. If your Conviction (After any Threshold Dampening)  is higher than the Natural Threshold Rating of a particular place (i.e. before any characters’ Bless This House are added in), You may make a Conviction roll, lowering the place’s Natural Threshold Rating by the number of shifts you roll above the Threshold Rating (To a minimum of 1). Multiple individuals who have this power can stack the effects, making a coven of those Faithful to Demons or the Unholy a force to be reckoned with. And woe to anyone who is so foolish as to invite one of them in.

Interesting then, that not a single thing in Our World has one. Additionally this would completely disallow characters like Kincaid or Macfinn. Unless of course we decide that a custom template functions in that respect.

I thought that was previously established in several threads regarding Scions.

Also, Under Total Refresh Cost, characters that receive the +2 bonus to refresh are denoted by a Template note: (Pure Mortal). This seems to signify that only Pure Mortals get the bonus, and that the extra Refresh in their Total is due to their Template being Pure Mortal.

I see most of this argument being derived from a stubborn refusal to include "Template" as a character trait.  With that, this becomes easy -- either the character sheet has "Template: Pure Mortal" on it, in which case the character is wholely non-supernatural but does get the +2 refresh bonus that is built into the Template, or the character has "Template: <anything else>" on it, in which case there's no +2 refresh bonus.

Even without having an explict "Template:" entry, though, every character does in fact have a Template defined on his sheet in the form of a High Concept, which is linked to Template. 

Umm, On the Character Phases Worksheet, there is, explicitly, a "Template" entry, which must be defined. Does that help decide things?

So why use it?  The PC's can't fight it.  They get to fail at protecting people or all die.

That plot has fail written all over it.  PC's losing is fine, assumign they had some chance in the first place or if the opposition had some awesome plot related reason they interacted with it. 

"Zombies?  Yeah we lost to a horde of zombies.  I shot them in the head a million times but I couldn't hurt them.  Oh yeah and they were throwing cars around...."

That a story you want your players telling later?

I think not.

Facing an opponent that is unstoppable by direct force is supposed to result in the players thinking up another solution. Just because Sancta came up with stats for it doesn't mean that every level of characters will be able to defeat it. "If it has stats, we can kill it" doesn't apply here. If you're aiming for more of a horror angle, the characters freaking out over the innocents that they had to leave behind is perfectly in line with the theme.

"Zombies?  Yeah we lost to a horde of zombies.  I shot them in the head a million times but I couldn't hurt them.  Oh yeah and they were throwing cars around...."

Should be followed by:

"...And then we managed to lure them into that lake, by the power plant. Almost lost Bill in that raft, but he managed to get to the other side before we let the power lines fall in..."

Or something similar. The book does mention that some adversaries are better gotten around than gone through.

How about another Self-sponsored magic?

SUPERIOR DIVINATION [-2]
Description: A master wizard can refine his skills in a particular area so fully that his abilities are qualitatively different from those of a normal mage. While normal specialization simply makes one faster and stronger and more effective, this level of focus provides entirely new capabilities. Martha Liberty, the reputed Spymaster of the High Council, is so practiced at Divination that Scrying a needle in a haystack is no harder than finding Waldo for her.
Musts: You must have both Evocation and Thaumaturgy with at least some specialization in Divination in order to take this power.
Sponsor: This magic is self-sponsored.
Agenda: As self-sponsored magic, this power lacks an agenda.
Evocation: Superior Divination does not provide any form of Evocation.
Thaumaturgy: Superior Divination does not provide any form of Thaumaturgy.
Evothaum: Superior Divination allows its user to cast Divination rituals with the speed and methods of Evocation.
Extra Benefits: You may use Divination to create a Scrying window that counts as line of sight for casting Evocation or Evothaum Spells, up to and including spells with a Shift rating equal to half of the Scrying window’s shifts of power (Not including Duration), Rounded up.
Note: Because this power requires Evocation and Thaumaturgy, it grants no focus slots and does not have its cost reduced when the user already has spellcasting powers.

Evocations can sweep large numbers of weak targets very easily.

That's why you start off with 1 or 2 of them, then start doubling. When the Wizards are down to 1 mental stress before consequences, they better start thinking and stop zapping at-will.