OP, I thought you were already ran the electric fan setup on your van w/o any issues, no?
If you are not lacking on power and you can't really see any difference driving the rig, why not just go back to mechanical setup? Easy and reliable. Our 3500 Express van doesn't cool (100%) at idle when it's really hot (like 98F+ ambient temperature), just like my suburban also with the mechanical fan. A little tap on the accelerator gets the AC ice cold again if I'm idling a long time like in a drive-thru. My wife drives the Express, and hasn't complained. Other than that, no issues.
Driving around the van will freeze you up though! The fan has like 4 settings. Even on the lowest setting, the blower blows way too much air.
Sometimes the kids complain the back is too cold.
It's strange that with all of those combinations you are still overheating though. What did the Duramax Express cooling consist of? You already tested the Duramax fan and it works...That was probably the hottest running setup on those vans with the turbos and all of that extra stuff for the diesel. If I recall, anything in the 8600 GVWR+ has had a mechanical fan. That's true for GMT400/800/900. I'm pretty sure even the newest 2500HD+ trucks (K2XX and the T1XX) are still running a mechanical fan...gas or diesel.
Disclamer: I don't know anything about E-fans. All of my GM trucks have the OEM mechanical fans. Only our Town & Country 3.6 Pentastar has an e-fan, due to the transversely mounted engine.