I'm sure the family, especially with all the people arriving at their home, knew they couldn't say she ran away because they had to know if they said runaway, they wouldn't get any help from the cops.
Maybe that's why the investigation, publicity, etc. has come to a screeching halt? LE now believes she's a runaway?
Exactly! If your idea here is correct, then they wanted help from the cops. (So they downplayed the possibility of her being a runaway.) If they were hiding some secret knowledge, that one of them had murdered her or sent her away, they wouldn't WANT any help from the cops. If they didn't want any help from the cops, that, to me, would have pointed towards guilt on their part.