2. I don;t thin kthe impedancewill matter a whole lot. All else being equal, go with 8 ohms.

3. You can measure your 206 rolloff and then put in an appropriate high pass only xover, with the requisite slope. IF the slope is > 6 db, it may complicate things in terms of phase.
But, at the frequencies you are talking about, it won;t matter. Jst put a 1.5 MuF cap in series with the 8 ohm tweeter to begin with.
-akhilesh