Not sure I understand the question. Perhaps you could post a sketch, including phreatic and/or piezometric lines, which may not coincide. (Link to do so appears just below the box where you will write the reply.)
Your calculations have shown the slope to be unstable, and if there really is zero cohesion, that result won't change by changing the depth to rock. You would have to justify existence of some cohesion to change the conclusion, and phi' isn't terribly sensitive to changes in density. Yes, it increases, but probably not enough to change the big picture.
Where is the phreatic/piezometric line? I assume there must be one, at or pretty close to the surface, since 2H:1V is only 26 degrees, flatter than phi', and the slope is statically unstable. Temporary cut slopes in unsaturated sand without pore pressure are routinely made at 1.5H:1V without problems.
Are the 100 pcf and 30 degrees measured values, or presumed? 30 seems pretty low for SW-GM, as does 100 even if it is the dry unit weight, not total. This is some mighty loose stuff if phi' of SW-GM is only 30 (unless it's all shaped like marbles)!
BTW - You can't drill or get test pits? And this is for your employment, not a school assignment, right?
The seismic analysis is another can of worms. Be sure you need the slope to actually be stable under dynamic loading, and that you can't tolerate some limited dynamic deformation once every 200 or 500 years or whatever.