Question #9:
A bitcoin transaction with 6 confirmations is usually considered to be secure. Why is it so?
Maybe this is the right answer.
There is nothing special about the default, often-cited figure of 6 blocks. It was chosen based on the assumption that an attacker is unlikely to amass more than 10% of the hashrate, and that a negligible risk of less than 0.1% is acceptable. Both these figures are arbitrary, however; 6 blocks are overkill for casual attackers, and at the same time powerless against more dedicated attackers with much more than 10% hashrate.
https://en.bitcoin.it/wiki/Confirmation#cite_note-1I will add that probability of reversing the transaction drop off exponentially with each confirmation. So it is always dependent on amount of transaction vs confirmation . If the amount of transaction is more than cost of reversing the transaction of 6 confirmation then merchant will wait for more confirmation.