I've heard using the LINE account is the best, stability-wise so that might be something you should go out of your way for. To make an account you need to install the LINE client but afterwards you can uninstall/ignore it.
Yes, there is - from Main Menu tap Cogwheel button in the upper left corner of the screen. Inside opened Options prompt there should be a blue button with 'LOGIN OPTIONS' text on it. Tap it and U will be offered with options to transfer Ur progress to different service.
Reminder: Switching from any login method to any other clears the save state of the previous save (e.g. switching from Guest to LINE, transfers Ur progress and then clears save on GUEST).