So many things to say here, but if this is the original Zoom H2, then the line input should not exceed .775 volts (peak I think!)
I used the Core-Sound -11 dB mini-to-mini attenuator for my successful recordings with it, starting AFTER I botched the meat puppets in fall 2007 by not knowing about the limited input levels on the ORIGINAL ZOOM H2. (not the N)
https://www.core-sound.com/shop/attenuator-cablesYou can test the channels of your cable at home - if you tap on an exposed cable end, it often produces a buzz, which can be used to verify left-right channel assignment at the very least.
many boards have control room out as RCA
The photo of the "full" board you show has a totally unused mono Aux 2 bus with all the mix knobs turned down. That could be employed to created a mono output mix in a pinch WITH THE TOTAL COOPERATION OF THE SOUND CREW.
If using the Zoom H2, there is exactly one best setting for input level, and I do'nt recall if it was 80/100 or what, but any setting other than that number would created a sub-optimal signal chain, either adding digital gain with a bit of noise or digitally attenuating, while adding a bit of noise...
Ozpeter and I "met" on the Zoom board so he might recall the correct answer faster than I can find it.
Good luck and let us know if you are using the H2N, it's rather different than the old "electric shaver!"
