You can extend cautions by manually throwing the yellow. Go to options and controls or something, where you're quick chat is. Then you can change one of the values to !yellow$
Whenever you press that button, the yellow will fly, or while still under caution with "1 lap to green" you can press it again to extend the caution another lap. As far as reducing laps under caution, I'm not sure. I have heard you can change the track type to be a road course under the hood, then it will only run 1 lap of caution. But this of course would have it's own side effects.
Regarding AI crashing, it really depends on what the issue is. It can be drafting_distance, braking_dlongpad(?), dlat_pad, ai_line_modifier, ai_squeeze, lp's, or a combination of everything. Try to explain the symptoms or cause of the wrecks as to provide a more accurate adjustment suggestion.