[rbullockv8]

Pic #3 looks good!

1. The front lip (showing facing downward in your first photo) is the only part visible between the firewall and cowl, so it doesn't impact the fit. I think the main problem is the combination of the forward bottom of that lip along with the center hump. And maybe it's just the center hump that's the problem.
2. I suspect removal of the back edge bead might make it easier for you to reinstall since you are squeezing it into limited space between the firewall and cowl. Just a guess. Hope you can try it.
3. You might be right about removing the bottom hump (facing upward in the first photo) to get to .21".

However, no matter how soft the rubber is, you don't get much compression when it's spread out over the top 2.5 - 3 feet of the cowl. You just can't compress it much across an area that wide. If I get under my dash, I can see some gaps where my modified seal doesn't, well, seal. Guess I ground it too much. But I'm not going to be driving this in the rain, so I'm not worried. Many other gaps/holes in an open cab pickup.

One other thing: The right thickness for my truck might be different for your roadster. I'm sure the firewalls are the same, but the body cowls are different. Therefore, you might have a different fit and results.

Finally, isn't it amazing how some dumb thing like this takes so much time!! :-).