Adire Extremis 6.8 Questions

You are correct about actaves. 

A steeper slope will generally reduce lobing issues. 

There is no way to tell you how to go about this as it really depends on how the speakers measure while in your vehicle.  There might be a crossover frequency or slope required that your processing does not offer.  And if that is the case, then you might have to tweak things to fix the problems that arise, which will take proper test equipment to pull off.  You are trying to set things in the electrical domain, however the speakers themselves have their own acoustic properties that MUST be taken into account as well.  It's like trying to draw a perfect diagonal line on an etch-a-sketch while your controling only one knob, and a blind person controls the other knob.  Chances are you aren't going to get a perfectly strait line.