Is encrypting then deleting the same as securely erasing?
To answer your question 1: You often see encryption mentioned when you speak of securely deleting the contents of a computer.

The reasoning behind this is that if you for example fully encrypt the disk drive on your computer (i.e. use the built-in File Vault encryption as an example) - then no one can access the contents of the drive without the password.

If you then encrypt the drive with a long, random password and immediately forget that password - then you have effectively deleted the data. If you don't know the password, and it is not written down anywhere - noone can access the data.

The caveats are ofcourse if your password is so bad that it can be guessed, or there's a bug in the encryption routines that makes it possible to recover data without the password.

Remember that:

(a) You must not store or remember the password anywhere

(b) You must not add a password hint or anything that makes it possible to guess the password

(c) You should throw away any recovery keys or similar, and make sure you're not storing a copy of passwords/recovery key on iCloud

(d) The password should be sufficiently strong (i.e. long and random)

To answer your question 2: If you have already stored the data unencrypted, you're out of luck. Just copying them into an encrypted image or anything like that won't replace the data on disk. Instead you'll be making an encrypted copy of your data - there's no guarantee that the disk will really delete the non-encrypted data.