Also when starting your clients, make sure you start the new account by itself first and make the character. Otherwise you won't be able to make or create characters due to mouse errors.
If you encounter the mouse pointer bug in a situation that you can't /mci you can either:
command tab to the instance you want, then command tab through all open windows stopping on the one you want again. The double command tab seems to refocus the mouse.
you can also: tab to finder, run expose and then click in the instance you want with the mouse.