Don’t worry about the impedance going up unless you are using a tube amp to power it. The only area to worry about with impedance is the 200hz range, but it looks to only dip to about 3.6 ohms, so it should be fine for a nominal 4 ohm amplifier.
As far as the z-offset, what waveguide are you using? I can only guess without knowing the waveguide and with it for that matter. But no matter waht, I wold venture it will be over 1/2″ But let us know the waveguide, so we can see if we can help with that at all.
As far as the crossover goes, my main concern is you are dipping the high end down too much. I personally would aim for it to be closer to 81-82db. That way you can get that high end. However this should be fairly easy to adjust with your resistor. If I were you, I would try a few different itterations.