I, fence sitter extraordinaire, would like everyone to consider the following. If you dont find it a valid possibility in your theories, feel free to read and reject.
******Conjecture alert on*****
Mark and Dylan may have been arguing about Dylan going to see his friends. Mark may have damaged Dylans phone and/or iPod. The next morning, Mark had no intentions of taking Dylan to see his friends and did not really try to wake Dylan up, instead he passively aggressively moved around making noise. Then he left. Dylan wakes up, is super POd that his dad left without him and takes off down the road or someone happens by and picks him up. Bad things ensue.
I think the above fits all the requirements, with the exception of Dylan not using the computer (it may have had a password on it he didnt know) or using the landline (if he had not memorized his friends numbers, the landline would not have been useful).
Now, assuming the above, or something similar, is what happened, Marks actions COULD very well be the actions of a person who does not admit that anything they do is wrong, is passive aggressive, is controlling and wants things his way, thinks everyone should pay attention to what he says because it is so important, and is, in general, kind of jerky (IMO). There are a lot of people out there who are jerky, but not killers. Marks actions COULD be the actions of someone who is covering up the fact that he puts his own needs first (he didnt want to be bothered taking Dylan to Bayfield that morning), got angry with his son and damaged his communication devices, wanted his son to magically want to be with Mark when he didnt, arrived home later than promised to show Dylan who was in charge, took a nap instead of looking for his son who desperately wanted to go see his friends because Mark didnt really want to drive to Bayfield anyway, and didnt notice that the backpack his son arrived with was no longer in the location it was the night before because that wasn't important to him. And he feels guilty and will not admit that anything he did had any bearing on Dylan being missing, because he thinks it will make him look bad and he cannot have that. He has to act like everyone would behave just like he did. And no, finding his son would not come first, looking like it wasn't his fault would come first.
****Conjecture alert off****
I could point to a half dozen people I know who would fit that description above. Making sure everyone knows "It wasn't my fault!" is paramount and more important than anything, including their family. And they arent killers, just really self centered, self serving, and selfish people.
I am absolutely not saying this is the ONLY reason why a person might act like Mark does. There may even be some reason that wouldn't make sense to me if someone tried to explain it. But its one I have seen time and time again. And one of the reasons why I sit on my fence. Leaning a little to the side.
Thank you for your consideration.