You can aim for the +4dB mark for optimal SNR levels across channel and master. It's not -18 because that's the overhead on the mixer.
The incoming signals should be line level (1.23V) for analog or whatever the digital level signal voltage is.
I don't know what the DACs are calibrated to and we're unlikely to get that data from the engineers.
The output setting level is for the digital output / USB output levels.