BBM - I would really wonder about about a couple of things in regards to this.
1) what kind of father would NOT admit after 5 1/2 months that he lied that he spoke with his missing son in the morning? Pride is one thing, but my Gosh this is your child! If you (in general) lied about trying to wake your son up in the morning, why!? Wouldn't you (in general) just come clean about it rather than mislead the nation?
2) Wouldn't LE indicate that there is a witness? They may, they may not. But IMO, someone seeing that MR was asleep the Sunday evening and either seeing or not seeing Dylan Sunday evening is a HUGE factor. That changes the entire timeline.
3) IMO, LE is not going to compromise an entire investigation by NOT disclosing the time of disappearance if it can be narrowed down. They would IMO state clearly, something such as, "We have a witness that has come forward, this witness has verified that Mark was in the residence at 10:30 pm on Sunday evening and Dylan was not present"
Unless the implication is that "another friend" is involved in Dylan's disappearance and nobody has noticed that this person is missing along with Dylan.
I'm also certain that IF there was a call to Dylan's phone after MR went to bed, the phone records would indicate so. I know that there has been discussion on how quickly various phone companies record calls on the online version of phone records, Dylan IIRC has a verizon phone, which from my experience, shows immediate accounting of calls incoming/outgoing as well as sms incoming/outgoing. LE would have had ample time to determine any incoming/outgoing calls by now.