Wow, I'm impressed! Unfortunately, I did not ask for a name. He sounded fairly young, if that can help any.
I called to find out what the ride height is supposed to be and how much settling they consider normal over time. And yes, he actually tried to tell me that spring height does not effect how close to the ground the frame sits. I believe that I convinced him of the correct answer (or he already knew, I hope).
The part about AFTERMARKET trailing arms causing problems really killed me.
I did get this info from him: 1/8" settling over time at the most. They are willing to check my springs, but I have to pay freight both ways.
I neglected the gas pressure effect because the car weighs quite a bit. However, just a few minutes ago I played with one of the original De Carbon rear shocks and it took about 30 lb to keep that one compressed (could be different for a new shock). I didn't realize it was quite that much. However, all of these shocks will "push" by some amount, so only the difference between different shocks counts, just like you said. (I don't know how big the diffence between Bilstein, Koni and De Carbon is).