After reading many posts that seek to explain MR's behavior as suspicious I would like to weigh in. My thoughts are my opinion and based on past experience which I am not able to discuss.
There are people in this world who do not react the way we expect them to in situations. How many of us have experience with having our life under a microscope while under great stress. MR may be normally an anti-social person or unable to articulate his feelings, especially when called on to do it while his son is missing. Like the majority of people, he is not trained to speak to the media, especially the type of media that is of the NG variety, suspicious, probing and relentless.
He may also be feeling tremendous guilt. He doesn't have to be the person who actually was responsible for his son's disappearance, but because he was the last person to see him alive. He may have other guilt because he left him alone, didn't check on him sooner, etc, etc. He may be having a lot of trouble dealing with this guilt and seeks to deflect it by throwing out snarky comments about his ex-wife, withdrawing within himself or avoiding the real issue, his missing son. He may also feel under attack because everyone is pointing fingers at him and rather than try to refute each one, he has basically given up and said screw it.
A reason why he may not have spent much time speaking out during the first few days is that he was in shock. This type of shock can do strange things to a person. They are unable to function or comprehend much of what is going on. Some are better able to deal with this, but most aren't. Also, it is important to note that while many agencies are involved in this case, they were most likely bombarding him with questions and directions within the first few days. For most of us, remembering what we did last night exactly is difficult, but imagine trying to remember when you have many people in your face asking you questions while you are worrying about your child. This could explain some of the confusion around his statements, his attempts to remember any tiny clue, and his focus on things like a missing fishing pole.
Now all of this is my opinion and is not intended to say he could not be a POI, but to try and look at another angle so the true facts are not missed.