I did check the link to the forum you mentioned. One fellow suggested not to use an adapter. Apparently he uses a cable - USB to Firewire, which of course is also an adapter. What can I say? An adapter is an adapter. However, he may have a point there, since I imagine the cable version might be a bit more stable, not sure. Otherwise I don't understand why it would be such a problem using the adapter route. They are not expensive, so if it doesn't work it's not much of an investment. I'm on Mac Pro 2012 which only has Firewire 800 ports, so I found an adapter for Firewire 400 (Nikon Scanner) to Firewire 800 (Mac Pro) and it works exactly as expected. My main issue is with VueScan. I have scanned some of my old Kodachromes on my Epson V-750 through Silverfast and so far I have not been able to produce as sharp a scan on the Nikon SC 4000 with VueScan. I'm hoping to find and correct the reason for that issue, because I do believe the Nikon would produce a better scan than the Epson. Of course Nikon dropped the support for the Nikon scanners many years ago, and even though Silverfast offers a Nikon version as well, it is very pricey. Guess I'll just have to continue with VueScan and perhaps eventually solve the sharpness issue I am up against. As far as the Canon route is concerned there seem to be some very positive reasons for trying that if possible. Nikon offers and adapter for some of there lenses for shooting slides. Since I shoot Nikon I cannot say if Canon might have such a devise as well, but I think it might be worth the search Paul.
Otherwise, good luck with your "darkroom" setup as you described it.
Gary