that seems very strange about the seat switch...
On this unit, the Spark plugs and the rest of the ign. circuit should be good for at least 2500 hours.
does this unit ever get the engine compartment steam or pressure cleaned?
My WAG [Wild as Guess] about this is that you may have a load of corrosion that has run down from the battery into the wire harness connection on the top of the ECM, which (without a serial number is not possible to be sure) is mounted below the battery on the frame. -Any- moisture in that (something like 86 pin) connection can cause WIFs [Weird Intermittent Failure], and I always recommend that we put a few dabs of Dielectric grease on the pins before reassembly to force and keep the moisture out of that connection. NEVER use any other type of substance in that connection. the voltages and current flows in those connections is pretty small (lots of 5 vdc sensor circuits). if the Plastic cover to that connection is damaged in any manner, you will for sure want to get that part fixed, as this will be only the first of many WIFs caused by even very little moisture in that connection.
I would also suggest you check the engine oil level to insure you don't have a bad seal on the hydraulic pump. [also just a WAG]. the level will be high if the pump is leaking from the front seal, as the hydraulic oil will be leaking into the engine crankcase.
The "rotten egg" smell is 'normal' if your engine is running a bit rich, as it is the catalytic converter doing it's job, but why activating the seat switch makes it happen as opposed to [say] having the hood down or weight on the seat is a bit of a mystery. I still agree with Kevin T [above] that you are going to want to change out all the coil packs, if this motor is in the 7k and above hours range, and they have not yet been changed, as these motors run pretty decently on 3 cylinders, and you may not even notice the missing until you are down to 2 cylinders firing.