You are correct that case 1 is the easiest.
It is very clear that Venerable Chakkupala did not see that he was stepping on insects since he was blind. In fact, even one with good eyesight is likely to unintentionally step on insects and other small living forms every day. There is no bad kamma created in these cases.
The other two cases are a bit more complex. In those two cases, the person was thinking about the issue and acted based on those conscious thoughts.
On one hand, being ignorant and doing foolish things that can hurt others is not an excuse.
– However, in the extreme case of an insane person (medically declared to be insane) cannot be held responsible for his/her actions. That is the mundane law, and the Buddha also declared that such a person would not accumulate any bad kamma (I don’t remember the specific sutta.)
I have some things to attend to this morning. I will think about the specific cases a bit more.
– In the meantime, I encourage others to think about it and make comments.