Gas mileage sound fine to me but it could be attributed to your other problems. Sulfur smell seams fine too.

5-6k rpm idle is insane, that's close to redline. I think its physically impossible to actually idle that high, the IAC can't open that much to let that much air in so I would say something is up with your throttle, maybe sticking. Think you have drive by wire throttle so maybe the throttle electronics are messed up.

1998 Honda Passport EXL V6 3.2L Auto SOTF 4WD
3" Calmini Lift w/ HD Track Bar, 32x11.5-16 Super Swamper SX, Aisins

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