"No funny stuff or I'll do the job" always sounded odd.. The "job" should be successfully hijacking the plane, in that phrase the "job" is blowing up the plane and people.. it doesn't make sense. The "job" in that context is a failure..
Portland sunset was 4:33 Nov 24, 1971..
Cooper wanted a night jump, the 5:00 PM demand plus turnaround is well beyond civil twilight end and about Nautical twilight end..
These times are purely technical based on the angle of the sun but due to the overcast weather at the time it would have become dark even faster after sunset. Astronomical twilight is irrelevant here.
Cooper planned a night jump.. otherwise he would have taken a much earlier plane.