Currently, the Mainnet pruned dB is around 350GB. That is supposed to decrease significantly after a new pruning mechanism is released. In the meantime, we suggest you have 1TB of free storage space. If you'd like to run an archival node it would need around 2TB.

Backing up your node is deemed as best practice. To do so check the following tutorial.

If you have users defined in the Keystore of your node, then you need to back up and restore those as well. Keystore API has methods that can be used to export and import user keys. Note that Keystore API is used by developers only and is not intended for use in production nodes. If you don't know what a Keystore API is and have not used it, you don't need to worry about it.

If you created the address via the node API then there is no passphrase, you created a single address. That is generally considered not a best practice. You should create the account using Core with a passphrase or, better yet, with a hardware wallet— for example, a Ledger.

This tutorial will show how to set up infrastructure to monitor an instance of AvalancheGo. You can check the health status of any node on the following page.

No, it does not but as a non-validating node, there will be no access to the mempool.

To add a node to the validator set, please see the following tutorial.

Nodes not staking 2000 AVAX or more will sync the chains, but cannot participate in consensus decisions. They can be used to post transactions and query information.

You will earn the same as validators minus the delegation fee set by the validator you are delegating to.

The basic instructions for setting up a DFK chain mainnet node can by found here.