I'm not a Case 1200 guru, but to paraphrase Sherlock Holmes, when you have eliminated all the other possibilities whatever is left must be the answer.  Looking at what you have one it seems to have to be one of three things.  1) the central metering system is "shorting" one row 2) something between the central and row units is blocking flow, or 3) there is a drive problem with the row unit.

I've been trying to learn and understand a green vac planter and haven't looked a lot at the new red ones.  If I understand how that planter works, there is a central metering unit that feeds individual row units which then meter and singulate the seed, correct? Is it possible to have something between the central unit and the row unit causing a problem?  If so, does seed back up in the tube or is the tube normally full anyway?

My gut feeling is there is a problem with the drive for the row unit.  Is it possible there is a gera in the drive for that row not quite correct?  Broken weld, key missing, etc that allows the chain to slip under use.  Another long shot is count the teeth on all the gears.  If this row has a gear just one tooth off it will affect the population and be virtually indetecable from observation