A couple of thoughts on this....
The cell phone thing wouldn't bother me EXCEPT for the fact that she was a full time caregiver to her three very young grandchildren. As a parent myself, I would not have been comfortable with the no cell phone arrangement as a safety precaution if nothing else.
The thing found in the house all that time later was something discovered while looking "on hands and knees." LM has said she won't say what this is out of respect for the investigation. My speculation would be that she thinks she found some kind of blood evidence. But that's just an educated guess.
To my knowledge the house has never been ruled in or out as a crime scene. In fact, LE have never said they are investigating this as foul play or that they suspect foul play. In the podcast, Sandra's family says that LE are so far unable to get uncooperative people to talk (or to get search warrants) because this is a missing persons case, not a criminal case. If that's true, I would assume that no one's cell phone location data has been analyzed yet.
On the lack of cellphone: As a parent, I’d insist on it from a caregiver. BUT - I may also weigh the risk of losing presumably free childcare against that. In my area, full day care is 18-24k per year per child. If offered free care by my parent, I might pick & choose my battles. JMO.