Fake transactions take more than a 50% attack to execute. Because the transactions simply do not come with the required signatures.

For rolling back transactions, a 50% attack actually convinces the remaining honest nodes. For invalid transactions, even if you have 90% of the network, the remaining 10% of honest nodes will not follow the wrong chain.

This means anyone could detect this happening. At which point faith in eth will plummet, and so will prizes.

If the validator starts reporting false results, it will get "slashed" by the network, and the validator will lose some of that 32 ETH.