To answer the question as to whether unsent messages can be seen in messenger, the answer would be a “no.” It is impossible to remove a message before sending it and seeing it.

Additionally, Facebook will retain a copy of any unsent message for a period of time to ensure the message did not contain any policy violations.

Facebook will keep unsent messages for a short amount of time in order to prevent harassers from using the option to cover their tracks. This isn't the end to Facebook's plans, either.