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What obvious fubar am I am missing?

I am thinking that the gotcha will be timing even if you rotated the part 180 degrees.
I will admit that i never did any thread milling.

Actually cut and paste the helix and rotate it 180 degrees in CAM.  Rotate the primitive, not the part. 

Using exact stop milling of course. 

I did just now think of one problem with my helix approach.  I have to join a line to the bottom (and probably the top) so that the cutter is not engaged with the part when it tries to retract if it starts at the top, or gouges its way down if it starts at the bottom. 

You calculate the thread height for 1/2 rev then add that to the starting point. Have done it several times for large threads. It will put the second start 180 deg out from the first.

Justa thought, (;-)TP

Makes sense. 

I'll shoot a video when I cut it and if it works I'll post it.