Nit, perhaps, but relying on third parties to interpret the source material, and only learning what is supposedly permissible from secondary texts without learning how to engage with and argue over the subtler aspects of primary texts, is not much of a life. If you want to say it's enough for most people, you must expect most people to accomplish very little.
Guess my religious background.
Protestant? (Based on the emphasis on a personal experience of the text, unfiltered by mediators).
Well, yeah—we would expect most Muslims to accomplish little in the space of Islamic jurisprudence as its a whole discipline unto itself.
As does secular law. Is it feasible for everyone to become a lawyer? Because that's the analogue.
> to expect so little in the way of questioning and intellectual exercise from the people who follow its tenets.
But that's just not true. You would only assume that if you were not acquainted with the Islamic intellectual tradition. Rather most people simply do not put in the effort that's necessary to develop the rigor and depth required to join the discussion. That's more of a comment on average human nature.
Practicing Muslims rely on books and texts that systematize this knowledge and provide it in a practical form for daily use rather than go to the source material (which can be overwhelming if one wants to find, for example, the ruling on whether a certain action in a specific context is permissible or not). Most Muslim children, as part of their basic religious education, learn the basics of Islamic law and practice from such a book. That's usually enough to go through ones life.
The ruling on representational art is based on this kind of derivation. There are some exceptions (e.g. for educational, security etc. purposes) but they're generally narrowly circumscribed.
Sunni tradition relies primarily on the Quran and 6 books of Hadith (of which the two you've mentioned are the main ones). Shia tradition has a smaller Hadith corpus because of theological differences about the reliability of the chains of narration of the Hadith and hence the derived rulings are very different in some areas. I've generally seen Shia works of art where prophetic companions, angels etc. are pictured but non-realistically (unlike Christian iconography). Some of them blank out the faces but it's not a tradition I'm deeply familiar with so I don't know.
My theory is that the huge emphasis on calligraphy and tesselations in Islamic art is mainly because of this. We don't have (many) paintings and sculptures of religious figures like in the Christian traditions.