You pointed out that fraud on the lightning network was impossible because the transactions are limited to the funds of the network operators. This is my understanding of how the network functions as well. However, if the operators are willing to abandon the funds on the network, they can do whatever they want to the funds a transactor puts in their wallets. In this way they can prevent transactions from being executed, despite that they will not profit from the transaction they have disrupted, and in fact will lose access to the funds they have on the network should they do this.

The network operators use their funds to execute pseudotransactions that avoid mining fees, and since it is their funds they are using, they can just lie and do something else than they agree to. The funds they have committed to the network so they can provide the pseudotransactions would be forfeited should they do this, so they will not do so without some compelling reason. But, one man's trifle is another's existential crisis.

Fraud is NEVER impossible, my friend.