Yep, I'm definitely on to something.
This has several implications.
1) The RootTemplate used for the replacement has an effect upon the NPC's disposition towards the party (Ally, Neutral, Enemy). This is likely what caused Boreas to not act like an ally for my other mod. At the very least, this is what caused the Script to not function in my earlier post- by replacing a neutral Template with an Orc (hostile), the intro battle was summoned.
2) Globals *can* be replaced in the Main Campaign. Before we've just been replacing stuff in the Data/Public folder. This is a change in the Mods/Globals/ folder for the specific level.
Edit:
So I think the next step would be to gather a list of all Spiders and which map/general location of where they reside, so that we can mod them into something else. And perhaps a list of the NPCs we know that are capable of Summoning Spiders.