I think you should reduce the amount you are paying. You seem to be overinvolved in her life....I am not saying you are being overbearing I just mean that you don't seem to have stepped back from being responsible for your dd in that you are supporting her with a huge amount of money each month, worrying about her childcare arrangements on her behalf whilst she seems oblivious. In the nicest possible way, take a massive step back. She will never grow up and take responsibility for her own life if you are always in the background doing it for her. As hard as it might be to watch her do things wrong, make unnecessary mistakes etc you need to.
First of all, reduce the money, maybe by £500 for a few months and then when she is back at work by another £500. Don't just spring it on them but have a conversation about how you and dh need to begin thinking about your retirement.
Second, stop laying awake worrying about the childcare. You've said you will do one day. If they don't sort anything that's their own problem not yours. Stand firm.
Thirdly, you mentioned your dh having a tendency to make offers that don't involve him, I assume this means he offers your services "Yes, no problem, Mum can do an extra day childcare" type scenario. You need to tackle this with him, make it abundantly clear he must stop doing this for you. If he does just say to dd "I'm really sorry, Dad should've checked with me before offering, I am unable to do this".
You don't need to fix everything any more, she's a grown up.