Hi Phil
I'll give this a go. Let me say you are asking fairly reasonable, technical questions but are skirting the edge of overthinking it
The response curves of the speakers are theoretical and will vary substantially once housed in the car. The graphs/numbers are probably based on an idealized scenario. IRL they will tend to have odd resonant behaviour that means you will end up tuning by ear to some extent. I got myself a plug in reference mic for the iPad which is helpful in smoothing out the crossover from sub to fronts, particularly, adapting the sub parameters to the front speakers and avoiding big resonances in the foot well. I use SpectrumView, but any spectrum analyser app will generally do, and you can probably use the built in mic at least for finding odd booms and spikes.
Your approach is sound, cut over at 80Hz, i.e. as low as you can match, and I'd pick a reasonably soft roll off, so that you don't get obvious transitions on sweeps. So if you intend to use the crossover features of the H/U, I'd use 6dB/octave.
HTH
Martyn