For questions 1 and 2, you can generate multiple addresses with only a single private key, these are called deterministic wallets. It can also be recovered using randomly-generated passphrases that acts as a password.
You are confusing a
private key with a
seed. They are not the same. A wallet uses its
seed to generate
private keys.
Also, the "randomly-generated passphrase" is not a
password. It is a
seed. You can use a
password to encrypt your
seed.