1) Ball of the feet - your legs shouldn't be straight when the pedal is at the lowest position, you want more like 30 degrees from straight (so your leg forms a 150 degree angle). Too low and you put extra stress on your muscles and joints, and I'm not sure what too high does, but it's not good either.. I don't have clipless pedals yet but that's my next move (pedals arriving this week I hope, will go shoe shopping this weekend).

2) I have a 7.2fx (mine is the smaller 17.5", but I'm shorter than you) and with the seat adjusted correctly, I can't touch the ground with my feet.. at all! I can touch with one toe while still in the saddle if I lean the bike, but usually I get up out of the saddle when I have to stop, and have one foot solidly on the ground since I find it easier to start quickly that way.

3) This is a big "it depends" On your strength, fitness, and just what you're biking over. I was told once upon a time to try to keep my cadence at least 60, but I find that tough sometimes.. and sometimes I average more. You don't want to be pushing too high of a gear and wearing yourself out, but too low and you'll feel like you're spinning your way to nowhere.