I agree that the lack of activity on the phone on Sunday night/Monday morning suggests something may have happened to Dylan prior to when MR left his house to run errands on Monday mornings.
However, I myself don't necessarily pinpoint the moment something happened right on the moment that Dylan & R's texting conversation about hanging out the next morning stopped. Just because when I read the transcript of the conversation between the two boys (someone posted it on here...) it seemed like the conversation between the two ended simply because it was over. It didn't seem to me that Dylan was cut off mid-conversation. IMO, most teenage boys end a texting exchange when what needs to be said has been said.. rather than saying-- "OK, bye goodnight I'm going to go play video games" for example.
I realize that by the time Dylan was reported missing his phone was completely off/battery out, whatever. But I don't think we know for sure that the phone's power was cut off right after his last text, right? Please correct me if I'm wrong.
This is all just my opinion based on my impression of the texts between DR & his friend R on Sunday night. If something did happen to Dylan on Sunday night or very early Monday morning, I don't know if it was necessarily right at 8pm.