1) No, you can use it for even 20,000 Hz if you wish.
2) To me, it is better to use the App+Rat. I put my minidsp 2X4HD back in the box and have not been able to find use for it. It had been used for no more than 2-3 months, will sell it for cheap if someone is interested.
3) Yes you can, in fact I did use it with Audyssey in the beginning because I was able to make some minor gain that way, but not if used just by itself.
Either way, if your goal is to achieve around +/- 1 dB 20 to say 120 Hz (1/12 to 1/24 smoothing), then I would guess a lot of trial and error is required, but in reality I believe +/- 2.5 dB should be more than enough, then it is a matter of customize one's preferred slope towards the low end. In that case, I could get it done in less than 2 hours, and that's being pessimistic.