Really distressing situation for you and your husband Debz, my heart goes out to you. I was wondering if you considered that the school incident had nothing to do with your daughters attitude towards you and her dad, it's merely a reason for her to justify her behaviour to anyone and everyone that will listen to her.
It was when you said about your injured foot and the trouble you had getting around for 10 weeks as well as her texting or just sending her son across the yard to your home. Apart from when she needs something , such as the wine the previous night, she doesn't come and visit , she sounds as though she has stopped seeing you as her parents, as individuals, as people of any worth except for what she has already had.
I'm not saying this to be hurtful but as the focus of all your distress is on what you think you have done wrong ('cos that's what she's said is the heinous crime ) you haven't noticed that her low opinion and poor treatment of you has been going on much longer. This is exactly what my daughter and her partner did to me, I was so upset and confused at the perceived unforgivable crime that it took me a while to realise that no, it wasn't that she was in a bad relationship, it wasn't that it was all something that could be resolved if only we could talk etc etc etc
I've come to realise that my daughter and her partner are two of a kind and both want the same things in life, I'm not part of that life, I'm no longer an asset or a means to an end.
Perhaps you need some space, seriously consider moving home which will relieve the daily pain and turmoil that you are in. If your husband is right and they are going to sell up, goodness knows who you have living so close to you. Leave any monies to your grandchildren in a trust for when they are older and try to make a good life for you and your husband. I didn't intend to write a novel sorry 