Well, it seems to be set up pretty well from here. You don't really have to use image points the way you are though, you can get the x/y position of the edges of the sprites in the expression editor.
As for the slope problem, you could use containers for your slopes. Make two different slope objects, both exactly the same size and shape. One will be your solid that toggles on and off, and one will be your continuous detector that you can check for per-pixel overlap. Since they're contained it will pick the proper instances so you can check overlap on the one and turn solid on/off on the other.