Hello Jan,
thank you for the feedback, maybe it's a DMX speed problem. The merger is regenerating the DMX signal, and the output is at the hight limit of the speed range. Sometime some fixture do not manage hight speed DMX, this may be your case. The good news is you can switch the merger to a slower output speed. Opening the Merger case you will be able to see a "Slow DMX" label with 2 pins side by side. Connecting those 2 pins will active the slow DMX function.
It's strange that The SDC-12 is having some issues with the merger, we used the same kind of controller for our testing here without any problem.
Could you tell me on which input of the merger you are connecting each of your controllers?
About the possibility to send OSC message from faders, the only one I know is our KIOSC app. You can download it on android, Ios, PC, Mac and Linux. You can then create a custom layout , in your case 6 faders, and set the OSC message they will send to control the LPU2. It will be a touch interface and you said physical faders, but it's the only solution I know for now. A more expensive solution would be to use an IOcore and some faders,,,
I hope this will help you,