That sounds like it's only running on one cylinder. Are both pipes getting hot? The fuel volume seems OK...so that's good! Now to figure out which cylinder is misbehaving....the SV doesn't run well on only one jug.
Do you have a temperature gun? That's the neatest way to inspect the pipe temperature, but a quick touch of the finger will work too...just be quick!! Some have sprayed water on them to see if one steams and the other not, but if you can isolate it to one or the other would be helpful.
Seeing as you have the O2 sensor, and there is a chance that it's gone south and WILL play hell with the light throttle running, if you get much farther into the diagnosis without joy you might unplug it to see if the running changes any.
I'm running the -9- heat range too....but have the excuse of increased compression ratio and believe the stock engine doesn't need that cold except maybe for extended Autobahn type running. You are correct in that the engine should run OK with the -9-'s though they will be more succeptable to fouling of course. I ran one summer on -9-'s with the stock engine and it was fine...so this problem lies elsewhere.
If both jugs are firing....then it sounds like it's getting excessive fuel. I know you said you checked the vacuum hoses...especially the one to the 'boost sensor'..yes? The manual has a check you can run on it to be sure it's not gone wacky which will make the injectors just dump the fuel and is on the 'possible culprit' list. SV1000S 2003 Service Manual
It's cumbersome to diagnose a machine long distance like this!
To review: You've set the TPS and it seems to be working correctly...yes? Even without the engine running the line '-C00' should move to the top with just a wee bit of throttle moving. A bad TPS could make the engine run poorly. A bad IAP sensor could also...so it might be worth a check with a vacuum gauge and voltmeter to see that it's scaling correctly when vacuum is applied. It should show .5-4.85V with no vacuum to full 28"applied...or the other way around I can't remember.
Are you sure the sparkplug cap won't come off? The coil end is potted with some epoxy or such but the cap just screws on...though someone might have added some RTV to seal the connection. It'll unscrew! Especially if you find one pipe colder than the other your original thought that it's spark related might very well be correct. With only one jug contributing and needing to hold the throttle open it'll mess up the working jug too so finding both plugs fouled seems normal in this case. See if both pipes are hot....then go from there. And let's unplug the O2 sensor just to take it out of the mix for now as they'll run just fine without it and the ECU will set a fixed idle mixture setting.