If, in fact, he already had S. for that entire week, prior to the murders, then someone had to have seen her with him. A neighbor seeing her outside on the farm, a store clerk sees them come in together, proof in pics on his phone, a FB post, or a pic on one of the other family members' phones. I'm pretty sure that CH was at the hospital, he'd probably know where S was that week.
If he didn't pick her up until that night at 10:30 then that should be fairly easy to prove also. For instance, KR2, might very well know. KR2 appears to be the last person to see her alive. KR2 never mentions S going with them fishing, but she did mention, that HMR's newborn stayed with HHG. If S wasn't with HMR, then HMR probably volunteered the info, for example; HHG is keeping K, and S wanted to stay too, or, HHG is keeping K but S is still with her father.