How does nonce really help to prevent double spending attack?
If there are 2 transactions with the same nonce and from same account, the transaction that is first included in the blockchain will be considered as valid and the other one will be rejected.
This is the concept used to override transactions in ethereum. If you have a pending transaction and you want to override it, you can send the same transaction with same nonce and a higher gas price (so that the later transaction gets mined). So if the later transaction is mined, the previous one becomes invalid.
Using nonce would relief us from 6 confirmations?
The 6 confirmations will be needed apart from using nonce. Using nonce you could prevent same transaction being included in blockchain.But there is a chance that the block in which your this transaction is included do not end up in the longest chain and hence you need to wait for some more blocks for confirmation.