He told me when we first met that football would always be his number one priority
Your DH sounds like a dick, why the fuck would you marry someone who said this to you?
you’re shirking your duties and exploiting other people’s kindness if you delegate it when you don’t have to
Using a willing babysitter is not exploiting someone's kindness if they are happy and it's not all the time. You've said yourself you've used GP's to babysit before.
I also felt so uncomfortable hearing him trying to guilt trip her and upsetting her when it was all for self interest
There was no need for this. A simple 'mummy's poorly and daddy has to go out so granny is going to look after you' would of been fine. There's nothing to negotiate, DD is a child and doesn't dictate your lives. I wouldn't of asked her, I'd of told her.
OP you've given a huge dripfeed at the end of your thread which usually happens when people start disagreeing with the op which if you included at the start, would have changed many of the responses you've had.
I still can't believe you married a man who told you football would always be his priority. What do you expect from someone who would say that to you?