So are you basically wanting to connect 8 audio streams from your DAW project to an analog mixer for analog summing or something like that?
I read through the manual and I think you just need to realign your thinking a little. Yes the headphone 1 and main L/R outputs parallel line outputs 1 & 2, and the headphone 2 output parallels line outputs 3 & 4, but that doesn’t mean you have to reconfigure anything necessarily. It just means whatever streams or stems you assign to 1824c outputs 1 & 2, those are the only things you’re going to hear in the main outputs 1 & 2 and headphone 1…same goes for anything you assign to outputs 3 & 4 and headphone 2. But so what? Don’t think of it as main L/R and headphones 1 & 2 are stuck to my output streams because if you’re connecting the 8 outputs to an analog mixer, you’re going to use the analog mixer for monitoring, cue feeds, etc. during your summing operation if that’s what you are doing. I wouldn’t be using the main outputs or headphone outputs of the 1824c if I was using it as a stem dump box…just assign whatever in the DAW to whatever of the 8 output channels, hook up the snake, and use the analog console as your mix and monitor station. You’ll have stuff that can pass from the main outs and the headphone outs on the 1824c, but you don’t care because when using the 1824c in the above type of application you’re not using those outputs of the 1824c anyway.
My 2 pence.