To what extent are certain principles of some Hanafis valid, which necessitate complete adherence to the madhhab, and which understand Abu Hanifa's saying, "If it is authentic, then it is my madhhab," to mean that his entire madhhab is authentic, and which consider all hadiths in the Six Books to be authentic, with the exception of forged ones?

There is no obligation to adhere to a specific madhhab (school of thought). Allah has not enjoined following anyone except the Messenger, peace and blessings be upon him. If the truth becomes clear to be contrary to a madhhab, then it is obligatory to follow the truth. The imams themselves forbade blind imitation of them, such as Abu Hanifa, who said: "This is my opinion, and this is the best I have seen; whoever comes with a better opinion than it, we will accept it." The deviation of scholars from a specific madhhab is towards the saying of another imam who is equal or greater, and they do not contradict the consensus of the Ummah. Weak hadiths were known before the era of the imams; the Salaf (early generations) were meticulous in verifying hadith and only accepted what was authentic. Not everything in the Six Books (of Hadith) is authentic; they contain good (hasan), weak (da'if), and even rejected (munkar) and fabricated (mawdu') narrations. The compilers of these books themselves ruled many of their own narrations as weak.