Probably because they have not been able to confirm yet one way or the other. May be talking to neighbors/friends to find out if she was seen somewhere else that evening. The AC guy has already been checked out and cleared. He is not a POI.
Little info about taking LDT's. LE will not usually tell someone immediately whether they passed or not. And if it was not conclusive, they are not going to say at all. I don't think they would tell them they passed if they didn't, unless they want them to feel comfortable and slip up somehow.
AFAIK, they do not suspect the bio mom at all, simply because she was an hour and a half away, and can prove her whereabouts, and because the little boy is still there. If she were going to take one child, she would take both. I don't think there is any hurry in giving her a polygraph, because she has been ill, and they are satisfied that she had nothing to do with it.