Morning Sac!
I'm not too good at the timeline, but is it possible that Howard administered the drug to Daniel before he went to Wendys/gas station? Like maybe he could've done it at a point in which Anna had dozed off and only the two of them were awake. Gave him a drink of some kind. Then he went to Wendys/gas station expecting Daniel to be dead by the time he got back. Just like how he left Anna alone and was MIA from the scene when she died.
Or maybe, although less likely, he could've given something to Daniel in the car on the way from the airport. Something that would take a few hours to kill him.
Does this in anyway conflict with the timeline?
They said that Danny had been dead for some time....So, say an hour before 9:30.....say 8:30. The timespan between last seen alive and death is 6:30-8:30 and I think that's MAX.
In Anna's case: I believe the time was the same because Taz said that 1 1/2 hours had passed when they checked her.
Those two times seem comparable.
Howard didn't know (that I know of) what meds Daniel was on and if he was taking them. He likely gave him something RIGHT AFTER the pictures to start the process of making him dopey. Howard's "He said he was sleepy (or tired)" tells me that. What Danny said is simply not an unusual statement.
More likely, he was reeling and starting to struggle with life.
Whatever killed each of them was a massive dose to "END IT"...what I call the killer dose and the imminent cause of death.
Short answer...Danny was given drugs earlier to make him unable to resist the later Killer Dose. Same as Anna.