I dont think thats a good idea. I think it will just spread in the attic. I've seen it first hand, and worst case scenario condensation will drip from the ceiling. Mold will go throughout entire attic.
There are a lot of variables, depends on how well your ventilation setup and outside temps, insulation, etc. Sounds like what you have a gable vent and is not the greatest.
Not sure you have an easy solution.
To answer your other questions, the shorter run with least amount of bends is best. Its best to intake at an ambient temp, from indoors. Indoors has slightly higher Co2, and indoors is more of a constant temp. I wouldn't grab from basement, to much humidity.