Silden, I think I need another pic to see what's happening here... I've been using my 2mm hex driver to set the spacing between the upright and A-arm. The manual calls for 2mm gap on the lower pivot ball but this is pretty hard to measure with calipers so I just use my 2mm driver as a feeler gauge.

You are right. I through the two pictures out without any real explanation.
OK.
The Tool engages in to the two slotted holes on the chassis
You can measure and set the distance between the upright to the outside with a caliper. usually with the Car upright.

In this way you can make sure that the width is correct on left and right side.
After that I measure the overall track width with a Ruler or Caliper.

The 2mm setting off the manual gives you a starting point, but the front and Rear arms on the EVO are identical and this results in the same width on F/R.
But the front should be 1 mm wider than the rear for good rotation. A good starting point is F 187 & R186 measured on the outside of the wheels.
We are testing/changing the Track Width for each layout. We go narrower on a really tight layout and wider on a flowing one.
That is the reason we made this Tool.

So I decided to be as scientific as possible last night and just made one change to the car. I left my 0K roll dampers alone and just changed the front heave damper. Last week it had 2K in it and this week I jumped to 7K. Right away the car cornered more aggressively and it was easy to go fast. I was pretty excited but of course after 5 consecutive good laps I had a racing incident and blew a tire (hate/love these fug'n USGT tires). I had to put new-ish front tires on for the rest of the night and the grip just wasn't there yet. LOL, I was so mad at myself for being dumb in the first qualifier.

Oh gosh my heart is throbbing for one of these now. I am a sucker for innovative ideas.

Is there a reason these are getting discounted? Is there another model about to come out? (I don't really care, I'm not going to race it - I just want to build it and play with it!)

There are already some options for the car. You can find them easily on Serpent website, the EVO ones, the springs, the solid axle outdrives, the pullrods, the hexagons, etc.

And there is something a little bit mysterious: Motormount center long for 4XPro. This one can be bought alone or with the full motormount kit.

I really don't know what's the purpose...

Indeed but I was not sure he was talking about your parts in fact... Mostly about existing one, the "must-have" that will make the bill heavier... JMHO

I would love love love a mid motor conversion for this car.