most of the time the regulator wont throw a code

Do you ever experience hard starting after the car has sat awhile?

No. Thats the problem. It runs like it just came off the showroom. Just stinks.

When you checked the FPR, did you leave it disconnected for a minute or two to be sure the diaphragm is not ruptured?

Check this video, at around the 1:50 mark.

GM insisting it is fuel with too much sulphur

I didn't know sulphur had an effect on MPG. Sounds like they don't want to spend time diagnosing it.

Which still leaves us guessing our heads off until either A. The FPR is verified, or B. a decent scanner that can show live data is connected to the car. Other than that, we can spend pages and pages of what this could be.

The local mechanic/ postman/drunk says the clicking noise is coming from the resonator

any chance that is where the foul odor is coming from?

No. The smell is coming from the cat converter because it can only process so much exhaust. Whatever it can't process, = sulfur smell.