That’s true, the windows version of Reaper have compatibility issue with the LR-16.
We already started to looking this problem. But the Reaper also available for OSX, and this version working flawlessly with LR-16.

You can set pad in the LR-16 MENU, and the device will be using that in interface mode and USB recording mode as well.
Windows mixer panel can be used for setting up monitor mix levels in windows. So the Faders in this application have no impact on the physical inputs leves senitivity of LR-16.