Your comment that the Qur'aan and Sunnah "offer no guidance" as to how to proceed with regard to politics and economics is simply completely, utterly and bone jarringly false.
There are certainly things that are specified by God and things which are left open to human choice.
This is the essential point of this whole argument. By separating 'religion' from all other spheres of life you are making a distinction that will fall down in many places.
Just as Qutb's distinction between the hard sciences and the social sciences or arts or philosophy, etc may also fall down as well.
I prefer, to both Qutb's and your way of phrasing this issue of what has been ordained by God and what has not, to simply state that a Muslim should be careful about any endeavor that he engages in, that it is consistent with the supreme belief in God and in the revelation that comes from God.
Revelation is a higher source of knowledge than either physical observation or the vaunted scientific methodic and certainly higher than the intellectual musings of philosophers or logicians, of whatever religion those people may be.
Sorry -- I shouldn't have included economics there, but I think the reference to politics holds up pretty well.
I actually regard the latter as a strength, not a weakness, by the way.