Jet emissions are
mostly carbon dioxide. Of course burning a gallon of kerosene produces over a gallon of H20, but it also produces about twice as much CO2.
No. Think about it. It only requires a small amount of water to push the RH from 99% to 100%. Then up to 99% of the water in the initial contrail comes from the atmosphere.
Actual ratios will vary with by the humidity, and the ration of RHW to RHI. But in the case of a persistent spreading contrail, over 99% of the
eventual ice budget comes from the ambient humidity.