Can someone try and explain what they mean by "splitting an account" being an attack vector. How would you "split an account"?
Splitting an account means dividing its stake into more than one other accounts, which are controlled by the same user of the original account.
The conclusion of the paper is that splitting the stake into more accounts reduces the chances to generate a block for same amount of coins. This was also discussed here Fairness issue on NXT and its solutions - #4 by petko.petkov . So splitting an account is disadvantageous to the attacker (but not significantly).
My question: would it improve security if all my active nodes had the EXACT same NXT balance? (I have an NXT clone running separately from NXT) and will announce an airdrop sometime...). Not really public yet.
No, it will slightly increase the chances of the attacker if both you and the attacker own a significant percentage of the total stake.