Is it permissible to use impure honey for treatment, and is that considered an insult to Allah's bounty, even though Allah stated that "therein is healing for mankind"?

It is permissible to seek healing through honey, and drinking it does not constitute an affront to the blessing. Rather, it is a prescribed act, just as it can be applied to limbs and used as an ointment. It is preferable to drink it mixed with water or with food. The fact that it may not be of the best quality or pure does not preclude benefiting from it, even if its benefit is less than that of pure, high-quality honey.