In answer to Question # 1, I believe that it doesn’t take much a change in the greenhouse gases in the atmosphere to start a chain effect. A small increase in CO2 and CH4, from human activity, can lead to a small uptick in average mean temperature. This in turn allows the atmosphere to hold more water vapor, water vapor is the largest greenhouse gas by far, which in turn causes an increase in temperature. A higher temp allows the atmosphere to hold even more water vapor and there is an increase in evaporation. Warmer oceans release more CO2. Basically, a small change in one or more of the greenhouse gasses can, over time, have a dramatic effect on climate as things cascade. I’ll answer Question #2 in another post.
The problem with that theory of continuous positive feedback from warming is that the absorption of infrared light by both CO2 and water vapor saturates at almost 100% at a certain concentration of both molecules. After the concentrations reach those levels, the molecules don’t absorb significantly more energy and the positive feedback essentially stops. CO2 is a very ineffective greenhouse gas and if the concentration doubled that would increase the earth’s temperature by < 0.24 degrees C: