(a)  Yes, that's right.  As to when the contract is made, and assuming Australian law applies to the transaction, the provisions of the Electronic Transactions legislation will apply.  The effect of this legislation is explained in section 4.11 of this module. 

In summary, when acceptance is made by email, acceptance of the offer takes place when the message indicating acceptance reaches the system (for instance the email address) designated by the buyer for communication, whether it has been read or not or, if a particular system (email address) has not been indicated by the buyer, when the communication actually comes to the attention of the buyer.

In the present case, the seller's acceptance has clearly reached the buyer and the contract has come into existence.  It is too late for the byuer to withdraw.