There is no "important mission" and there never will be. Unless you are doing something like map placement, or specific character order, there is no real way to screw up (except for throwing out essential mission items)
Urrgh. The Cheetahs is a plot mission for god sake. In order to advance to plot (and get more missions) you are gona have to do it soon. And if missions disappear they come back at random (just as they came in at random).