Unfortunately, I believe the Thunderbolt2 to FW800 adapter is from a Thunderbolt (PCIe protocols), not USB protocols. Trying to run in reverse would require something that is Thunderbolt capable. The NT2 is USB3, not Thunderbolt. Big difference in this case.
The one problem with Thunderbolt3 using USB3 TypeC cabling is that they are not the same thing. Yes, a Thunderbolt computer also has USB3.1 within the Thunderbolt connection. But many devices that are Thunderbolt3 require a Thunderbolt connection to run. Thunderbolt IS PCIe, which is why most any PCI device can be run on a Thunderbolt connection. Only USB devices can be run a USB3.1 TypeC connection though.
molṑn labe'
"I am a mortal enemy to arbitrary government and unlimited power. I am naturally very jealous for the rights and liberties of my country, and the least encroachment of those invaluable privileges is apt to make my blood boil."
--Ben Franklin