Nabutso answered below so I removed some inaccurate information.
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Also, why do I need so many different usernames and passwords for one game? Main, alt and mule? What is a mule anyway?
You don't. An alt is just for if you want to level another character. Some people like to experience it all over again, other people level alts for other reasons like crafting. A mule is just a character people use to store stuff on. They are completely optional.
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There is no guide on the website.
I feel like I've seen one, but that may have been with the old way to install (it was a real pain in the ass. This one is super simple) so they may have felt it not necessary to. I dunno. I'll have to see if I can find it anywhere.
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I had to open up all the files until I found one that may or may not have been the actual game launcher. Just a huge mess and not user friendly at all. There is a shortcut on my desktop that does nothing.
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You should be able to install, double click one icon that's automatically added to the desktop and that should open the game where you enter your username and password and you're in the game.
That actually is what it does. You double click NasomiXI Launcer and it brings up this:

From there you click an option and it lets you log in.