This may or may not have been premediated, there is always the possibility that this was a crime of opportunity.
"Getting to know Mollie" means that law enforcement is trying to understand her habits, personality, relationships, and various other things that help them to understand what is normal for her. Victimology is standard aspect of any missing person's investigation. It in no way implies that she was kidnapped by someone she knew, although that's a place they will start at.
Social media could have made her an easy target, as it may have given away the fact that she was alone.
They said that they have no reason to believe the public was in danger. This does not necessarily mean that they know this was personal, or that they know who did it. They just don't know.
The dogs were put in the basement by Mollie, no one would have had to take care of them in the time period that elapsed.