In short, if you have that sort of reasonableness requirement you are leaving it to the common law to figure it out, and the courts are going to have to figure it out. It's not the first time that such language has been used and the common law has been left to figure it out. There is uncertainty to start with. Most of us have faith in the common law to come up with the right answer in certain circumstances, and the beauty of the common law is that, on a case-by-case basis, it goes through this.
I find it hard to resist the urge to answer the previous hypothetical question. I think it's tied into this. Certainly in principle it is reasonable. However, I think that the hypothetical problems with mistaken identification and lack of accountability are inseparable from that question, which in principle I fully agree with. There should be a corresponding right. The problem is the lack of accountability that flows from that, the uncertainty in the language, and the possibility that a citizen's arrest is going to be met with violent resistance, where the arrester is not properly trained to carry out such an arrest.