As to the idle issue…
Have you check / tested fuel pressure?
Might test the fuel pressure regulator to see if it is functioning correctly.
You state: It’s also losing oil albeit very slowly; no puddles under the car, but new oil is around the oil filler cap at the end of a day of driving.
This may be due to excessive crank case pressure.
Suggest doing a compression and leak down test to see if your dealing with excessive wear in the combustion cambers.
Concerning the exhaust smell:
Obviously there must be a leak somewhere.
From what you describe, my first thought is somewhere around the firewall area.