Also, the rep said that they got the file from other agencies, who had been mailing my old address for 15 years (yea right). But even so, it would be reasonable to assume that the "alleged" debt was reported to the credit bureau's long ago and has since expired. So if they report it again, then I figure they get sued.
Yup. I've sued four of these companies and in three cases they wrote me checks. In the last one, I let the suit die on the vine & the court dismissed it for want of prosecution. I also sued Equifax for an outdated entry from my college days that they refused to delete.
These people respond to lawsuits. The general strategy is: bait them into violations of the FDCPA and/or FCRA. Document those and then sue them. They will settle quickly as their legal bills quickly surpass any profit they would make by collecting from you. Win or lose, you aren't liable for their attorney's fees.