I remember thinking I'd try Simple Green years ago thnking it would be more eco friendly. But like you I found that it just doesn't do the job as well as solvent. And yes, oil actually is an excellent cleaning agent. The thinner the oil the better and faster acting the cleaning. Up to the point where the oil is known as a solvent like kerosene or mineral spirits.

I also disliked having to rinse the chain with water to remove the SG and then having to use WD40 to displace the water before I could apply the lube. So I bypassed the whole issue and use solvent for cleaning and oil for oiling.

As for eco issues I found that the solvent would settle out and leave a "tea" of clean solvent with some oil in it with a layer of grunge on the bottom. I'm able to use this settling trick and reuse the solvent almost indefinetly. The only loss being what drips on the garbage bag I use as a drop cloth and what gets wiped off the chain with paper towels. A gallon lasts me a little less than year for bicycle chains, motorcycle chains and many other shop uses for mineral spirits so I think that's pretty eco friendly. The REALLY grungy stuff goes into a solvent bottle and on to a paint and solvent recycling depot.