And excess water vapor can be quickly re-equilibrated out of the system through precipitation. It is considered by scientists to be more related to feedback than direct forcing.
Yes, a much, much worse greenhouse gas.
I can name probably a dozen chemicals that in teeeeeeensy tiny doses can do great damage. Your personal incredulity at small numbers doesn't really mean anything.
And, again, on planet earth where I live when excess H2O gets into the system it can quickly get out. It is called the HYDROLOGIC CYCLE which is VERY different from the CARBON CYCLE which is needed to remove excess CO2 or CH4 from the atmosphere.
And, again, this has been shown to NOT impact the temperature trends. Studies like Peterson (2008) showed no statistical impact of urban heat island effect on the overall data trend. And even NOAA's study in which poorly sited stations per Anthony Watt's selection, were removed from the data and it was shown to have no effect on the overall trend.
I suggest you do more literature reading.
It has been EXPLICITLY studied repeatedly. Again, your literature review needs to be updated.