Here's a image of Phobos from MGS in 98 showing a better view of Stickney.

Now here's the deal with rolling boulders. Do you see how the "grooves" wrap up and around the crater rim? With the extremely small and variable acceleration due to gravity on Phob, I don't think this would be possible from rolling boulders.

Also, you mention that the fractures should be radial and you see here that they are indeed that. Both concentric and radial fracturing is observed from terrestrial impact craters so I wonder if in a smaller body impact, radial fractures dominate. I guess the hyperbolic fracture on the left flank of Phobos could be a quasi-concentric fracture in the small body case.

Your absolutely right about layering, your case of igneous layering is a good example and another would be metamorphic gneiss and schists. Where did you see that?

As for Phobos looking sedimentary, it is that on the surface, kinda... Any body in the plane of the SS without an atmosphere acquires sediment sized dust on its surface giving it that softened look. This accumulation of dust is called regolith, for those who didn't already know this.