Hello again daalias.
Thanks for starting a separate thread after our exchange at http://discussions.apple.com/message/20807945
Your new info is clear and helpful. Well done!
Based on your new topic info, I have some ideas additional to my previous suggestions:
(1) When you tested multiple FW800/FW400 adapters, did you try both of these?
One of them solved the iSight problem for mini user "upcut" as reported in this topic: http://discussions.apple.com/message/20772929