The Declaration of Independence has no more to do with allocating duties or powers to the Federal Government than does the Gettysberg Address, the novel Tom Sawyer, or Bill Clinton's first inaugural address. In this country, we have chosen to organize our Federal Government and to limit its powers by way of a written Constitution. If you feel that the states have become too powerful and that the Federal Government is without adequate powers and influence, the Constitution even includes procedural means (Article V) by which it can be amended.
In this particular case, the court found that Congress had not authorized the Attorney General to utilize the Control Substances Act as a Federal weapon with which to interfere with Oregon's assisted suicide law. In that regard, the judge pointed out that in 1998 and again in 1999, the Congress considered amending the Controlled Substances Act so as to enable the Attorney General to use it as a weapon against assisted suicide laws, but that on both occasions the efforts failed. Rather than again seeking Congressional approval, Attorney General Ashcroft decided that he didn't need (read couldn't get) any additional or specific Congressional approval and so he just declared that he already had Congressional approval and proceeded on his own to disrupt the operation of Oregon's law. The judge simply ordered him to stop.
This is not a case of the "state authorities" dictating to the Federal Government "the interpretation of federal law." Attorney General Ashcroft's efforts have been thwarted by an "interpretation of the federal law" by a member of the Federal Government's own judicial branch.
I would hardly lump all of those together, because obviously they have various levels of significance to our nation. The DOI has much to do with allocating duties or powers to the Federal Government, as it is the document, formulated to correspond with our independence, which articulated the principles behind our separating from Britain and having a Federal Government of our own in the first place. Neither the Gettysburg address nor any of Twain's novels can make any such claim. Without an independent nation, there would be no federal government. Therefore your argument that it has "no more to do with" allocating powers to the federal government is specious at best.
The rest of your argument, as is part of the court's, raises valid concerns but is unconvincing. Specifically, you mention attempts to modify the statute to specify the medicinally illegitimate use of a controlled substance to purposefully kill someone else. While you and the court acknowledge that the current Congress could not pass the amendment, it ignores the question of whether or not those who passed it felt, given the term "medicinally legitimate," was even necessary. You know as well as I that the courts have ruled pretty incomprehensively over the history of our nation, and has so often exceeded the powers granted it that it has become the norm rather than the exception. In this case, their ruling is inconsistent with itself. Only if it ruled against the constitutionality of the Controlled Substance Act could they legitimately claim that Oregon has the authority to determine what is medicinally legitimate. Instead, what happened is that the court has allowed the state to determine the meaning of a federal statute independent of anyone else. A federal statute without the means of interpreting and executing its provisions is not a federal statute at all.