Dang! You've been busy! It is going to take me a while to get through all your info as I only get a few times a day that I can get on here and post. Here is a brief stab at your numbers;

You actually would want a car that would tend to be more towards oversteer than understeer. From the factory these cars understeer substantially and you want to loosen them up some to get a better "feel" to them. To steal a line from the movies, loose is fast and on the edge of control. These cars tend to work best on that edge.

Obviously there are a number of ways to achieve 75% roll couple. The MP book list a combo of 1.0 t-bar with a .85 sway bar and 230# leaf springs as 74.31 roll couple. There is another 1.0 t-bar, 1.0 sway bar and 275# leaf spring as a 76.8 roll couple. So you can vary how you reach your percentage based on the parts available. The speed and track at which you are racing at would determine if you want to go higher or lower with springs rates. Flatter tracks or street driving could use lower spring and higher sway bar rates. Higher banking or autobahn use would require higher spring rates and lower sway bar rates. Keep in mind that those numbers were from competition ready cars driving on super speedways and front to rear bais that was 50/50 or more to the rear, so don't take the as a firm recommendation of spring rates but rather an example of different way to acheive the same percentages.

6.85 Cg height does seem low. You might have to refigure that one. I would guess it to be 12-15" plus or minus a couple inches based on you lowered ride height.

yes, your roll couple of the sum of you wheel rates from torsion bars and sway bars and springs all combined.

In the mean time, here is some data straight from mother mopar;

torsion bars and wheel rate *old nascar sizes*
.92 - 137.5
.96 - 163.5
1.0 - 191.5
1.10 - 263
1.14 - 325
1.18 - 349
1.22 - 425
1.46 - 875 *
1.66 - 1455 *

sway bars and rates *these are from kit car data, which is a bit higher than stock sway bars as they used a splined shaft with bolt on arms.
.90 - 295
1.0 - 450
1.062 - 572
1.125 - 721

leaf springs, spring rate and roll rate
96# spring 70 roll
120# spring 87 roll
180# spring 130 roll
275# spring 200 roll
400# spring 290 roll
526# spring 380 roll