1. Yes, there is a soft cap, but not in this sense. There is always a hard cap for every arena size - no matter how low you set the price, the attendance will never exceed this cap. For example, 20k size, max = 19.5k. In your case, 21.8k size, max = 20.16k. Arena size below ~19k-20k, max = arena size. This cap would increase as you expand your arena further. But it would be under 100% of arena size once its past ~19k-20k. In this sense, it is a soft cap. I still haven't found a formula to determine the hard cap for each arena size. More examples will help.
2. Not anymore because it's very likely that your prices are too low.
3. Same as 2.
You should fine-tune your prices so that your attendance is slightly under 20.16k. (assuming you don't expand your arena anymore)