"Event script referenced an item that does not exist."

Well, for one, check every character's default equipment (to make sure there's no problems, clear them all, then reset back to whatever the default should be). Sometimes you might end up wearing a potion as a hat if you change the item type. Also, make sure the monsters involved are dropping actual non-deleted items. Inventory items (if you're loading a saved game) might also be messed up.

I see no problems with the actual code beyond figuring in the zeros as actual numbers and THEN dividing by 8 even if there are 3 monsters. To make sure it isn't a programming bug (which I can't tell just from the code given), make a new game and don't change anything except for adding your script. If it works fine, it's something with how your project is set up.

As to escape-proofing, honestly I would just set a switch within each needed monster group to disable running within your event command and have an auto-event within the map to turn the running back on after the battle. No calculations needed really, just disable running from the monsters you want to (if you really, really want to do running based on level, just do a simple branch that checks to see if Level < 15 to disable running for that specific monster group)

At least, that's the theory - I'd check it, but I'm on a lunch break and don't have time at the moment to test it.