I saw your post before, but I didn't respond because I myself have lost hope on fixing a hoarding situation, though with other complications.
A couple of alternatives to CPS: you could try the county health department, or the police in your area might have a hoarding task force. Those generally only apply if the hoard is unsanitary, damages the home, or extends outside the home. Also, anything like that is likely to escalate any existing tensions. If you've already involved CPS, can I assume that there are some burned bridges with your daughter?
Hoarding is often associated with ADHD and OCD; are those present in your daughter or son-in-law? Sometimes treatment for comorbid issues can function as treatment for the hoarding. At the very least it can start them on a path to see things from a different perspective.