I don't see any way to get the 4'/100mm mid into the dash location.

I don't feel very good about getting 3" dome mids into that location, actually. The 28mm Dynaudio MD100 tweets have about 3mm of clearance all the way around, and they are 62mm across from edge to edge. That's about 68mm, or 2-7/8".

DLS makes a kit with a 6" midbass, a 2.5" dome mid, and a 28mm tweeter, and I think I'd have to grind the deges of the dome mid flange to get THAT in there.

That's basically why we used a 28mm soft dome with a low mechanical resonance and crossed it over at 2500 - to fill in the front vocal presence. You can do that with DLS, Dynaudio, Morel, a/d/s/, and some others.