there is actual physical evidence--skirt, placard, money--that are very problematic for the FBI Flight Path.
This evidence seems to support the FBI flight path, not dispute it. The skirt (although never confirmed to be from Cooper's plane, but I'll humor you) was within the official 8 mile width for the Victor airway path of the FBI. The placard was also found within this 8 mile width. The Tina Bar money find could be explained by the current of the Columbia from where the jetliner crossed the river. So, I reject the premise that the physical evidence is "problematic" for the FBI flight path.
Also, while human error and expert fallibility are possible, you are asking us to believe that at the height of the Cold War, the military and other government agencies LOST TRACK of a large plane. Not just anywhere but an area of the country with high value military and industrial targets along our most northwestern coastline. These targets include (but are not limited to)the nuclear facility at Hanford, Seattle-area Boeing plants, the Puget Sound Navy Yard, Fort Lewis, and McChord and Fairchild Air Force Bases. This air defense area was on high alert for Soviet bombers constantly in 1971. The idea that all of these eyes and ears, while on full alert, somehow got the location of a hijacked airliner wrong borders on the preposterous.
In summary, the physical evidence you cited isn't problematic for the FBI Flight path (on the contrary), and the idea that not just one or two people erred, but the entire military-industrial complex failed at a simple task at which it prepared for constantly seems highly improbable.
For the record, I'm happy to hear any respectful, reasoned, logical arguments to counter what I've stated.