Of course without knowing the cause of the conflict or how bad the insult was we can't understand your husband's feelings. If they 'made up' I don't understand 'he won't allow her to visit'. It sounds a bit childish and if it's upsetting you I'm surprised he isn't more supportive.
Equally, if it bothers you, why not ask your mum why she never asks after him?
Regarding the money-if you addressed a thank you card from both of you I can't see the problem.
Weddings are always tricky, but presuming he loves both you and your daughter, he will understand or even expect that your mother is invited and if he's not asked to make a decision about that, it's probable there will be no issue. They can avoid speaking to each other without it being noticed unless a big fuss is made. Hope everything goes well.