No problem about the support. It's our job, and what you pay for. Thank you for your patience and helping us solidify the 3.0 release!
Yes, you could avoid this if you deleted the users in CB. That could cause problems with existing classified ads as they won't be tied to an owner, so I don't want to lead you down the wrong path to cause you other problems. As an alternative, we could help you add a few lines of code to change that error to say something like:
"The email associated with this Facebook profile is already used on this site. Please log in with that account and then use the Facebook connect login button to join those accounts".. or something similar. If you'd rather just blow the users away, that's certainly easier for us
The login address in the JFBCLogin module is set as a parameter in the login module itself. On this site, we actually leave it as the default "--Select Item--" which leaves the user on the same page they tried to log in with. Otherwise, you should be able to set it to something else and get it to redirect properly there.. if not, you know where to find us..