The key is the data table, representing which each character encodes to or from.

In the code in this article, the key is the mapping stored in ‘CIPHER’.

First, second, and third statements of the provided source code.

That's not a requirement. Pigpen is a substitution cipher.

I think a lot of this depends on if you read the article as the scream cipher being specifically the exact listed substitutions or just any substitution with forms of As. Also depends on how you define encoding, cipher and the overlaps between the two. Plus questions on the relevance of intent, transformation of data, plus changing of meaning and definitions over the years. Some people say morse code is a cipher, but braille isn't - definitions can depend on way more than the black and white logical "but it does this" you're using.

You'd do better debating this with a real life friend over a pint, rather than wasting your time trying to argue with multiple people here.

Sweet Lord Jesus.

Interesting, I actually expected it to encode a single letter with infinitely long combining marks such that 'highlighting' it was just highlighting one character.