You go to background and make a background with your username on it using a program such as "Paint" for PC's.
No, the owner has no way to know who is watching the videos they put on youtube.
Use the underscore
YouTube username is always your original name. The Gmail username is the ID which is used to log in. Then it is your name that shows up.
just the username. not the password
No.
Alex Day's YouTube username is Alex Day. He previously used the username nerimon.
I can't tell you on this but go on YouTube and put in supergenorpg (the persons username) and look for it or look for it on his channel.
osnapitzari
JedwardTV
You can't change your username on YouTube, though you may create a second account with a new name.
Miley Cyrus's youtube username is mileymandy