The transactions of e.g. Ethereum have a fixed order becaue they are in a blockchain. Because IOTA stores it's transactions in the Tangle, we don't have a sequence. In order to execute smart contract consistency is required. To achieve something similar on IOTA, mandatory timestamps are necessary.
You can already send ...
The problem here is, we can't prove, if the timestamp is correct or not.
Setting the timestamp back to 01.01.1900:
Case 1: The node is honest and rejects the transaction, because the timestamp is not correctly dated -> everything is good
Case 2: The node is NOT honest and distributes the transaction further.
Now other nodes have no way of knowing, if the ...
You have to distinguish between bundle hash and transaction hash here.
The attachment timestamp is not encoded in the bundle hash (which is signed by the spending transactions). Otherwise it would not be possible to reattach a transaction (with updated attachment timestamp) without double-spending. This is also true for the nonce, the message/signature, the ...