^ I agree that where certain settings are located in the menu is not always intuitive. It just struck me reading this that if you have it set to record in stereo mode, then it should automatically reset that stereo pair to hard panned left and right, and that would avoid accidentally messing up a stereo pair recording. To me, you would only use panning in the recorder for isolated mono tracks like a boom mic or orchestral spot. I know that professional mobile sound people expect the ability to mix right on the recorder, but generally they're using something much more high end with a dedicated control surface. With something relatively simple like the 70D, I think it makes more sense to just record your mono iso tracks centered and then adjust panning in post.
Being that the 70D is directed at video / DSLR users, I suppose they included this so you could quickly make a downmix of 4 > 2 channels to send to your camera.
Good point with respect to the second paragraph, but with respect to the first paragraph, I'm not sure we're on the same page. Maybe we are.
I might be wrong, but I don't think the pan setting has any effect on how tracks are written to the SD card, only how you hear the files when you play them back through the 70D (and probably how they're routed to the headphone when monitoring and, as you point out to the mixdown output). For example, he said he was getting levels on both channels. Unless I'm wrong both channels should be written properly to tracks 1 and 2 on his SD card. That said, when you play the recording back on the DR70D, if the mixer sends both recorded channels to the left channel on playback, then you won't hear anything from one channel since they're both panned left...so they're doubling up on each other on the left channel.
It's also my understand that the only thing 'stereo' does vs. mono is that it causes the DR70 to write two tracks onto the SD card for each file, where mono writes either two or four separate files, one per track. If the mixer panning has both channels panned hard left, a stereo file will still play back with one channel having sound and the other silent.
Please correct me if I'm wrong...this is how it works on the Zoom F8 mixer which I've become WAY more familiar with than the DR70D.