1. Not really that hard, depends on the exchange you're want to use. I think the crucial element is privacy because if you've share your privacy in online, you will not able to become 100% anonymous since either the database or someone have save it.

2. In short mining is a process to verifying and solving Bitcoin blocks, mining can maintain Bitcoin security because the higher the difficulty and the more people mine it, this will make people who want to perform 51% getting slim.

3. Because it's decentralized.