Sorry, but you have made a series of unrecoverable mistakes; There is no option to change or modify a username in any way, there is no option to reopen a closed channel, and, finally, there is no option to use a name that has already been used before. While YouTube will "remember" what capitalization you use when you create a channel, the letters are the same no matter what capitalization you use, so that name, in any combination of capitalization, is no longer available.