Ok..just me again with another .02 comment...
IF your DD can get this house to close, then your DD has to realize that that house and everything that happens in that house from that moment on, is her liability. If someone stays in that house and lets say, falls and hurts themselves, your DD is liable. This is another reason why renting back to them, is not a great idea.
When people start acting goofy, like these sellers are, I wouldn't trust them to act appropriately. If there is a divorce situation, or if money is some sort of issue, or if they're resentful that they're getting "forced" into honoring the contract and selling, you don't need the possibility of them dreaming up ways to stay in the house or causing any kind of monetary/liability to the new owners. People get goofy over things and get irrational when things aren't going their way. Another reason to get them out immediately IF you can get this house to close.