Tom, W8JI wrote:
"A traditional directional coupler works by comparing voltage across the
line at any one point to current in the line at that same point."

Almost. It compares a voltage sample to a current sample, both of which
have been converted into d-c voltages. These have been carefully crafted
to be exactly equal d-c voltages regardless of the power level in the
line.

I`m giving up on correcting line by line.

Important fact is that a reflection reverses the phase between the
voltage and current produced by a wave.

So when the samples from the forward wave are siummed, their total is
exactly 2x the value of either the voltage-derived sample or the
current-derived sample.

When the samples from the reflected wave are summed, being equal but
opposite in polarity, they add to ZERO. Calibration is so the total
produces the correct value on the power scale for the wave in the
forward direction.

To get the power in the reverse direction, the input and output are
effectively exchanged so that the forward power indication cancels and
the reverse power indication is produced by the sum of its voltage and
current d-c sample outputs.

Best regards, Richard Harrison, KB5WZI