How can I fish cable through a 90 degree turn?
Your "intake vent" is better referred to as a "return air vent in the stud wall cavity". The reason there appears to be a 90 degree turn in it is because there is! Behind the drywall, at the base of the wall, there is a hole where the floor would be, this transitions into a metal duct which travels back to your HVAC unit, just like you see if you open up your floor registers; the air is just moving the other direction.

I would not drill a hole for your Cat6 cable in the attic OR the basement in this stud cavity for this reason. Rather, pick an interior wall stud cavity where you have basement access (try the utility room) and there aren't preexisting electrical runs. Drill a hole from the attic and basement (alignment doesn't need to be perfect, just hit the same cavity). Use a paperclip &amp; string from the top, and magnet from the bottom, to fish a line between the two. Then use that line to pull your Cat6 cable through.