Negative Balance in Raiblocks?

It seems that it's possible to have a spender with negative balance or even incorrect balance? The whitepaper doesn't clarify how the following ledger attack is prevented?

How exactly is the balance of a Raiblocks account verified?

Who verifies that balances are not negative? Is it the representatives that somehow should be monitoring all transactions?

It shouldn't just be the receiver, since the sender and receiver might collude.

So why wouldn't the sender and receiver collude and the sender falsifies the balance?

So what happens when a negative balance is detected? How would the network reject this?

The whitepaper mentions that the sender is allowed to modify their own chain. So what happens when the sender spends coins they don't have?

The whitepaper only mentions "If it is a send block, the balance must be less than the previous balance".

Also, who makes sure that the sender decrements the spent coins correctly? What prevents the sender from sending 10 coins though only subtracting 1 coin from their balance?

It's also noted in the wiki Since send blocks contain an account's balance, in order to calculate the balance from the block chain, only blocks up to the last send need to be kept.