Does Multi_Signature transactions enable trust-less payments? (Edited)
In the paper, it mentions a third party, Chuck, who will resolve disputes.


  (1): If customer (Alice) before receiving the product, signs 2-of-2
  transaction, then transaction is payable for merchant (Bob), but what
  happens if merchant (Bob) does not send the product to customer
  (Alice)?


Assuming Bob only cared about getting paid, he would always sign a transaction which paid himself, so it does little good to require his signature. If alice signed a transaction which did not pay her, it would show good faith, if there was another party involved, Chuck. Chuck would require that the product be sent to Alice before the funds would be fully released to Bob. In this case it would be a 2-of-2 multisig script requiring Alice and Chuck, and requiring Alice to sign before Bob ships the package, for example.


  (2): What happens if customer (Alice) denies to sign 2-of-2
  transaction before receiving the product, and after receiving the
  product also does not sign the transaction?


Assuming this is a 2-of-2 requiring Alice and Chuck from above, then Bob would require Alice's signature before sending the product, that way the arbiter, Chuck would be responsible. Note that using a multisig script on blockchain to trace an external physical action would require trust (from both parties) in the arbiter, Chuck, and is not completely trustless.