Though I think it might be useful to others that it appears the immobilizer kicks in several seconds after the ignition is first set. That means, if you immediately try to start the engine, it will fire up and then stall. If you listen to the fuel pump to prime and then start, the time required for the immobilizer to be triggered will pass and you will get a crank but no start situation.
The rough idle issue is a bit more interesting. I went ahead and replaced the spark plugs with Bosch FR7DPP+24. I was thinking the car was going to have some old Volvo OEM plugs, but what I found inside the engine was really so much worse. I'm surprised it ran at all. The plugs were really not that tight too, just above hand tight. What the hell was the previous owner thinking? I regret not doing this earlier. Car feels so much better now, like day and night. Take a look at this garbage that I found in my car (the gap can be measured with a ruler and is somwhere around 2 mm):
Some googling shows that the Bosch FGR line of plugs fits a lot of performance engines, i.e. E36 M3 and some Porsches. I guess it's good for n/a engines, maybe not ideal for turbo Volvo, especially when they're way beyond their service life.
