How can both of your statements be true?
If the CO2 is trapping it, how is how is it water vapor released by a one-time event has a relatively short lifetime in the atmosphere? In any event, what eruptions may be a "one time event", there are many that erupt for years. Over forty are erupting now.
Global Volcanism Program | Current Eruptions (si.edu)
And why is the stratosphere cooling, at the same time the stratocaster is hotter than ever?