I feel for you. I have a similar issue with my old fridge (that I now use as a backup). It just has an off smell about it and I have scrubbed that sucker thoroughly but it's still there. I even called my appliance repair guy and wanted him to come out thinking maybe there was some part going (the fridge is 13 years old and has never seen the service man) but he was insistent that something must have spoiled in there and he never did come out and take a look. Told me to adjust the temperature. Given the age of the fridge and how clean I know it is I'm still not convinced something isn't going on it. Sounds like you've been pretty thorough in investigating all the possibilities. Maybe you can call a repairman and/or the place you bought it from to see what they think? I'll be very interested to hear if you're able to trace the source of the problem.
Another thought occurred to me...I know you said that you've pulled out all the appliances and looked underneath but maybe you want to look again. One of the most Godawful smells I've ever encountered in places I've lived or worked (and you said this was horrible) was the stench of some little critter (Door mouse? Gecko?) who'd breathed his last behind some item of furniture. Do you possibly have a dropped ceiling and something might be mouldering away there? Or any loose woodwork near the fridge or other appliances?
When I stand before God at the end of my life, I would hope that I would not have a single bit of talent left and I could say “I used everything you gave me.”