I'm with you on the woofer section, it doesn't seem like there anything that would roll off that quickly without a lowpass. I assume that means you would skip using any coil to encourage roll off when replacing the door woofer?The interesting part of the graph is where the two traces cross at about 320 Hz. I'm 99.8% sure that's indicating a crossover. It looks just like every other speaker pair I've seen over the years where there's a crossover between them. If there wasn't a crossover the woofer in the door would roll off at a much lower rate than that.
I'm still thinking that I need to use a single cap centered around 600hz for the dash speakers. If there is a factory cap, it's probably for protection at least. Is there any harm in adding a cap to a new dash speaker if the single is already highpass coming out of the amp?