Here are a few answers for you:
1) The FOX Family Double Dare run was shot first, then the 2nd half of the syndicated Double Dare run was shot afterwards. The glass wall was one of the few set changes from FOX Family Double Dare that remained when the syndicated Double Dare resumed.
Disregard any significance with the helmet colors, as the white & colored helmets coincided with different versions, respectively. Also, remember that the contestant uniforms for the 2nd half of the syndicated run returned to the same style as those worn prior to the FOX Family version.
Concerning the "Inside Scoop" clips, I do remember both "Inside Slop" & "Messiest Moments" VHS tapes were filmed/released at the same time in 1988. Without whipping out the tapes (sorry, lazy night, lol), I distinctly remember one of these showing a clip of a physical challenge in which one contestant held 2 fake legs and stomped "grapes" (balloons) inside an elevated transparent plastic box, but the spout would not work properly...towards the end of the challenge, the side of the box holding the disfunctioning spout broke-off completely, with all of the grape juice flowing onto the sitting partner contestant. This clip features the glass wall behind Marc's podium, which appeared for the 2nd half of the syndicated Double Dare. You are correct that there are no video clips of FOX Family Double Dare shown in both "Inside Scoop" and "Messiest Moments" tapes, but notice that the box art (front AND back) for both of these VHS tapes show photos (I believe shot primarily just for the box art) taken on the FOX Family Double Dare set.
2) I'm of no help on this one, sorry.
3) If I recall correctly, I'm positive the Orlando eps with the teal blue lights were shot first, with the amber lights shot afterwards.
4)This is the information I have retained for quite a few years. It doesn't add up to the "525" number that has been circulating online for years, but at a total of 481, it's as close as I can get:
DD (1986): 65
DD (1987): 65
SSDD (1987): 26
DD (1988): 130 (65 pre-FOX Family DD, 65 post-FOX Family DD)
Fox FDD (1988): 13 (entire version)
SSDD (1989, Philadelphia): 50
SSDD (1989, Orlando): 50
Nick FDD (1990): 40 (One of them being the Salute to Double Dare special)
Super Special DD (1992): 2
Nick FDD (1992): 40 (One of them being the hour-long series finale Tournament of Champions)
I hope this helps to somewhat answer a few of your questions. Hopefully one day, I can wrap some more research around getting closer to seeing how accurate this "525" episode count is!