I was re-reading Battle Ground again and something leapt out at me.  In Chapter 6, right before they get dive-bombed by one of the flying squids, Mab nodded to Ebenezar and said, "Br---"

I would take that to mean you need to think about it as rolling a 20 sided dice. If you roll a 19 or 20 it works anything else you are at zero or less meaning it does not work.  In other words you have a 1 in 10 chance of pulling off the spell.  Guessing a bit but it may well mean 9 out of 10 times the vampire gets to tear you apart because you are so tired from the failed spell you have no chance and had to be near the vamp to cast it.

I think Jim said it was rolling a -19 on sanity by using the spell.

Just randomly throwing it out, but my most likely suspect is Ivy.

The quote goes into detail about how the person would have to have intimate knowledge about the plans for LC, how to bypass the wards, etc. - almost as if they could read and perfectly understand the written wards and plans as they were put up.

Also, Ivy was probably about the right age at that point to read Calvin and Hobbes.

Plus, Harry gave her a name.  It's been made pretty clear that that is a Big. Deal.

Presumably, Harry and Bob went over written plans for LC as they were designing and building it - since it's written, Ivy knew about it, and since she is the Archive, she would have known about the flaw.  Because she knew what would happen if the flaw weren't fixed, she fixed it.