After doing some rodding at the aforementioned restaurant, I found that a whole section of four inch pipe was clogged with grease, (nope, no grease trap for the back sink!). And sombody in their infinate wisdom had made a loop basically allowing the flow from that source to flow into the main line via a two inch. Somehow, there's a pipe leading out the back which I got about 13 feet into before deciding, once again wet, smelly and with attitude, time for a pro with a pro machine. The little motorized unit I was using wasn't making appreciable headway. The tape just kept wanting to twist and what I got off the tip amounted to impacted fecal matter or clay. I'll hear by the end of today if there's a break or if the plumber can punch it through. Meanwhile, as the four inch pipe has been cleaned out, continous flow has been restored (it was spilling over upstairs when the sink was drained) to the back sink and the air dam problem by the ejection pump doesn't seem to be an issue. I'll still get some pics but my lithium ion batteries that I got last summer for the camera need changing.
I've found a better word for getto plumbing, it's called Chicago.