For inversion agitation with one roll in a two roll tank it's preferable to include an empty reel to take up the extra space. Otherwise the reel with the film is sliding back and forth and possibly causing surge marks at the sprocket holes. Perhaps a bigger issue for stainless reels vs plastic. It's also a good idea to fill the tank to capacity even for one roll, also to limit excessive surging. The extra air if you don't should not cause any issue though, consider that for rotary processing, a tank that can hold as much as a liter upright might be used with only 3 or 4 ounces on its side rotating.
Agitation is one of those things that is very sensitive to individual technique, and what works well for some people doesn't work at all for others. The real answer to problems caused by agitation is to change what you're doing, and see if that helps, if not, try something else. Once you find success, do that every time. What I outlined above works for me, ymmv.
FWIW, the twirling sticks work, but the swirling should be gentle and slow and in alternating directions.