For example, the model I am working on will need shape keys for blinking, adding a shape key is simple enough but turns out when I set the key to 0.8 the eyelid is clipping through the eyeball before falling into place on 1.0 where it looks normal, so how can I edit the shape key at 0.8 so it doesn't clip? every time I try to go back on the range slider and edit all changes are added up to the maximum range and so it would affect how the shape key looks at 1.0, since the model has to be exported to work on a game engine it needs to have single shape keys to assign on certain functions.
Shape keys can be used to change a mesh shape with a certain amount of vertices into an other shape with more (or less) vertices. But for the sake of (my) argument lets assume it is the same amount of vertices for the start and end shape. The same way i think you are using shape keys.
In that case i believe a shape key can be seen as a recording of two location coordinates for every vertex in the mesh. (a recording like an animation keyframe). One for the start shape and one for the end shape.
I think (when the shapekey Value is changed from 0 to 1) an individual vertex moves from its starting position to its end position traveling the shortest possible distance. In other words, it moves in a straight line.
If i understand (or imagine your problem) correctly, you are essentially asking if a vertex (part of a single shapekey) can have a third coordinate location (like an animation keyframe) along the way. A third location it would be at if the Value is somewhere between 0 and 1, and that is not somewhere on the strait line between the start and end location. I'm pretty sure at the moment it can't.
Probably there is some other solution to your problem, but i think you would have to approach your problem (and possibly phrase your (new) question) differently.
Hope that gives some insight : )