912 ULS with 460 hours built in 2008 1000 RPM drop during Mag test

So it appears you DO have a Soft-Start.

According to the chart, the 966727 modules switch from near TDC to well BTDC (normal) after a 3-8 Second time delay.

YOUR 966726 modules switch to normal once the engine reaches 650-1000 rpm. (NOT timed.)
650-1000 is well above cranking speeds, so it would be safe to assume the engine is turning under power.

Re-reading back from the beginning to see what we missed.

You bit the big one and purchase a new trigger coil Harness to no avail.
In your photo of the harness, I see 2 connectors, each populated with 2 Blue/Yel and 2 White/Yel wires going to the 4 trigger coils.
This leaves 2 open positions on each connector that should be occupied by the Brown MAG wires which we have pretty much eliminated, and (Wait for it!) the RED Charging Coil wires.
Have we ever looked at Charging Coils???  They follow the Mag switches whenever doing Connector swaps.

Pull Off the Trigger coil connector and measure from the RED Wire to the engine Ground.
They should measure 3.2 to 4.5 Ohms.
This is nearly a short and a hard value to measure accurately, but they should at least both be small and the SAME.

Or just swap the RED wires ONLY and see if the Problem switches sides.

The Ground side of the Charging Coils are independently screw lugged to the Engine and could be loose/corroded.

The Charging coils are the Power Source for the Ignition Modules and ultimately provide the energy that produces the Spark.

Hey Bill.

I did swap the two red power wires from the stator, like 3 times.  The problem did not switch sides.

Well, NUTS!  Again. ???

What the heck is left?

At this point I have ZERO clue.
I am not an A&P, nor am I master engine builder, but I believe that the tests I have performed were the correct test.  I am dead in the water for 2 weeks now.

Thanks everyone for your suggestions.