When Chanel #2 is tweeting, her message is more than the standard 140 character limit.
When Chanel #2 is texting the Devil, her phone shows the text bubbles in green and his phone shows the text bubbles in blue. Since they are both using iMessage, both should have text bubbles in blue.
It is impossible to receive a message from "Unknown" unless that is the name entered into the iPhone's contacts (which is unlikely in this situation) - the sender would be identified by their phone number or email address.
When the Devil is shown reading the text from Chanel #2, he is holding the phone upside down.
The first time that they show the Devil's phone screen while he is texting Chanel #2 there are multiple text messages back and forth but when they show Chanel #2's phone screen, she only has the two messages that the Devil sent.