Q1: My event is set as "Force Login = True" so why do I have a Default Participant in my event?
Although the portal will force the user to login, it does not prevent that user from registering someone else who is not in MinistryPlatform. When this happens, re-assign the Default Participant using the Assign Participant Tool.

Q2: How can I send an automatic message to a specific User when someone registers for an event?
This can be accomplished through a Process. MP comes with the "Form Response Submitted" Process which notifies the Primary Contact of the Form when a response is submitted. If the default Process does not fit your needs, you can always create a Process that does exactly what you are looking for! For more information, see Processes.