Not sure I can help with a solution, but I share your frustration with the scsi/usb problem. I have an agfa duoscan HiD scanner which has a scsi connection, and I use it with the Adaptec Usbxchange adapter that came with it (when I acquired them both 2nd hand some yrs ago). This works under windows xp, but nothing newer. The scanner would work on linux or on a mac using vuescan, but the problem is getting a driver etc for the usbxchange adapter. I have read about a patch for linux, on the internet, but lack the knowledge to use it. It may be possible to get the adapter to work on some mac operating system, and then using the scanner with eg vuescan should be no problem.
This isn't my thread, of course, but if by chance anyone reading this knows how to get the Adaptec Usbxchange adapter working under linux, that would be great!
All best,
John