Hey, great write-up! A few minor remarks:

The private owner key is your master password for the account.

The private owner key and the master password are actually two different things. Each key (owner, active, posting, memo) can be derived from the master password. Master password and private owner key therefore have the same "power". While the other keys are visible on the "permissions" tab, the private owner key is not visible on steemit.com. In any case, as you say: don't use the master password to login. Keep it offline and only use posting/active keys for login.

Presently, these keys do not have a primary function on Steemit. Therefore, you may save the private key offline and not worry about it further. It is believed that this key may serve a purpose in the future on Steemit, but for now, you won't be needing it.

Memo keys are used for encrypted transfer memos. Starting a transfer memo with "#" as the first character will use the memo keys to encrypt this memo. The transfer memos can then only be decrypted by the sender and the receiver of this transaction.