If the chain operator hides the block where step 3 happened from Alice (refusing to make it available), what can Alice do?
And how is Bob supposed to know that the fraud happened if he doesn’t check the previous Merkle state roots? Particularly since there’s no guarantee that there was an “Alice” defrauded in the past—this could just be a fraud on Bob, where Eve and the operator will reveal the prior fraud after Bob has been tricked into accepting the coin.
Also, does your mechanism depend on Alice noticing this unavailability and exiting within a fixed time period? If so, it loses one of the main advantages of Plasma Cash (avoiding mass forced exit).