I'm not really understanding when you talk about spring rates and shock travel. I'm not an expert on this stuff, but maybe you are over complicating it? Are you going seriously track the car? Maybe all of that matters then, but for the street, probably not.
I have the D2's, and on my car, i can barely push the front or back of the car down, its really stiff. So i think 4 inches of shock travel is fine. Its not going to bottom out.
For the springs, again i am not an expert on suspension tuning...but for mine, i tightened the bottom spring perch nut/collar by hand, until i couldnt tighten it anymore. Just hand tight basically. That will give me a "0" preload. A lot of people make the mistake of compressing the spring to lower the car, but actually you are supposed to adjust the threaded shock body itself. It says that in the instrctions. I think this is why some people complain that the car is too bouncy. And from what I have read, compressing the spring does not change the spring rate at all. So i guess the spring rate is preset and cant be changed? Maybe someone knowledgable can chime in.
Not 100 percent sure if you are talking about the bearing betwen the top plate and upper spring perch, but that piece of plastic is actually a bearing. its plastic, but still a bearing. I recently disassembled both my fronts completely after 6k miles to clean everything and check for knocking noise (turned out I had not tightned the lower locking collar enough, or somehow it worked loose over time). I am happy to report everything looks great. Regardless, if we are talking about the same bearing (i think there is only one anyways), all the components have no issues. i installed my coilvers in 2019, so unless D2 has changed something since, you should be fine.
After welding, I had the spindles/hubs powdercoated, and installed new bearings, races and seals. Rattle can would work, but nothing beats the longevity of powder coating. I know i took some pics, but I cant seem to find them...
Since I just had everything apart, I wish i had noted some measurements, really kicking my self about that. Its been 5 years, trying to remember, but when cutting off the old shock from the spindle, I left 3 inches. I think most people were saying leave an inch and a half. But with 3 inches, that still allow plenty of adjustment for the threaded shock body to go into the spindle/hub when lowering the vehicle. Unless you wanted to really slam the car, then maybe do 2.5 or 2 inches. I also did 3 large button welds, 9, 12, and 3 o clock positions.
Honestly i havent even adjusted the dampening on the shock yet. The car rides great, no complaints. I think you will be happy, just keep going and take more pics.