2.) I don't think Trump is being reasonable. People are relying on debates to see what the candidates think about certain issues. If a candidate is not there, people won't get his/her persepective on these issues. Also, it shows to people that Trump will back down even if he is offended by one person.
3.) Yes. Trump is very unpredictable and sometimes acts very rash. I don't think he really considers what his actions/plans