I don't think your daughter could pretend as well as she is if she were in real danger. She has to know you are there for her but that she doesn't do/say anything seems to be mostly her doing. You know she wants to be a nurse; I think you would know if she wanted to leave.

That her in-laws probably provide food and supplies is good; they are keeping an eye on the situation too. Anyway you can get their names/address and visit or call them? You might get a better picture that way.

The 9 year old is going to make "random" comments; kids do. I remember being with my niece and her making up a fantastical story that one would have thought she was mentally not all there but that's what children her age do. Think about yourself too, and what you "think" seeing things (like driving by a cemetery; you may think, "How pretty" or "I don't want to die") but you have better "control" over whether you say something or not; he's just learning. The "my father's a good father" thing does sound coached but if you have seen no evidence of bruises or broken bones, etc. or looks of fear, I wouldn't worry about physical abuse although it sounds like there's a lot of mental mess going around. But I'm sure the son-in-law knows you don't like him; how would you expect him to act, knowing that? He doesn't have a job, money, he has your daughter and you as in-laws not liking him; he's going to appear defensive and hang around to see what's happening, just like you are wishing to know what's going on in their lives.

I think you're "helping" as much as you can. I don't see any immediate problems or desire on your daughter's part to leave. If you can, I'd try to contact the other parents, see if you can get more information and/or give them money to funnel in (since it appears to be accepted from that quarter) or whatever. But there's only so much you can do from 600 miles away? I would keep in as good a contact as you can and let everyone know you care, get your husband to try to sort of make friends with the son-in-law, etc. I would try to give the sense of being friendly, relaxed rather than concerned. No one likes to think they're the object of pity or worry?