Such a Power would basically be All Creatures Are Equal Before God, except free and integrated with Evocation. Actually it would be a bit better than that, since it would negate Speed and Size and since monsters tend to have worse Discipline/Conviction than Athletics/Endurance.
ACAEBG costs 3 or 4 Refresh, plus a FP each time you use it. Add maybe 2 Refresh because the new Power costs no FP, and another Refresh for hitting Discipline/Conviction and ignoring Speed and Size...so you get a cost of 6 or 7 Refresh.
Mmm, but aren't Speed and Size powers costed on the assumption that they don't help with mental attacks?
Endurance and Athletics are probably more common, but not overwhelmingly so, I'm not sure the difference is that major (especially since this power would probably be mostly found on higher-refresh characters, and at those levels spellcasting arguably pulls ahead more.) Humans, especially True Believers/KotC and practitioners, may well have better mental defense skills than physical. According to OW, Red Court Infected, Red Court and Black Court have equal or better Discipline than Athletics. Also, ghosts, spellcasting monsters, faerie nobles...
Also, really powerful mental attacks already exist. 3 refresh in Incite Emotion gets you a Weapon:4 mental attack, 4 refresh for ranged, and it only relies on one skill rather than 2-3 like spellcasting.
Still, you have to buy into that separately from physical attacks. So a high Refresh cost for mental evocation is legitimate... but it's also unworkable character-build-wise, and I don't think a Molly-type character using mental evocations should be unworkable.
So what about some kind of compromise Power that allows you to make mental evocations at a cost in shifts, but has a lower Refresh cost?
Something like....
MASTERY OF MIND [-2]
Your evocations can affect the minds of your foes.
Mental Attack Magic. When you cast an evocation attack, you can reduce its Power by 3 shifts to make it deal mental stress instead of physical stress.
This does not allow you to perform psychomantic thaumaturgy effects, like reading minds, as evocation - you are still limited to evocation's options for effects.
Sleep spells are probably physical,
It... certainly would make explaining how it doesn't break the Fourth Law easier, but I really don't think so, since Harry talks about sleep spells being "grey magic" mind magic.
Now, maybe weaponized sleep spells work differently from regular sleep spells, but that seems counter-Occamian to me.
and Corpsetaker's stuff is probably thaumaturgy.
I think you really need weapon values (and probably pretty high ones) to get the overwhelming mental attacks she's got without making her a more powerful character than the Merlin.