I skimmed, I admit. But I think the long and short is that you and your fiance need to be specific and firm. "Mom, I have the pool to close. Don't come before 1PM." Or "We have plans with other people today. If you come, you cannot stay past 3." And if they say something at 2:30 about staying, "We told you we had plans and had to leave at 3. It would be rude of us to not be on time to our friend's house. Please respect our schedule." Catherine C also has a good point about doing your thing when you need to do your thing. They'll learn to come on time if you don't rearrange the universe for them.
It will only be an argument if he allows it to be one. If she huffs and puffs, he needs to stand up for himself/his family and say, "If the time we are able to give you doesn't work for you, we can see you another day." Rinse and repeat. No cajoling or arguing or trying to placate her. And though it's passive aggressive, if they would show up anyway, then DH stays home to do the chore and you and the baby stay out somewhere til the time you told them you'd be home. Or you ALL go out til the time you said you'd be home.
If the round trip doesn't work, then it doesn't work. "Mom, we've discussed before that we are not making the trip unless I can get a Monday or Friday off. No amount of guilt will change that. I'm not discussing this again." And then don't.
It may take time, but if you keep boundaries, most people will adjust to them, even if they don't like them. I also think it sounds like his SF is teachable and he might be a good ally - maybe take him aside if she tries this again and say, "We really can't have a repeat of the other weekend. I know you tried to get her out and I appreciate it. Is there a signal or something we can give you so we all leave at an appropriate time?"