If the groom says his mother can be difficult, it's likely she would be difficult in some other way if they moved the wedding up 6 months. I would encourage the happy couple to ignore the negativity, if groom's mom wants to pout or make non-supportive statements. I get that the close dates make things a little more hectic, but it's really MIL's loss if she's going to bellyache about it and not share the joy. It doesn't sound like anyone has asked her to be in charge of anything, is that right? It sounds like the engaged couple is solid and content and in control of their plans, and that's all that matters. If some of the guests can't make it after traveling to the other wedding, I'd bet your DD and future son in law would understand and if they are concerned or bothered by that, that is on them for planning the date they chose. My goodness, they sound very intelligent, surely they have put thought into this. I'll bet your DD's future MIL comes around. This is a lot, even good events, it's a lot, and it can cause anxiety. I think she'll come around at some point. It doesn't matter if she isn't involved in all the planning details. In fact, it's probably better that way. Less conflict, and she can just enjoy the wedding. I hope your DD isn't too upset about this. I think you should reassure her and not worry about it so much yourself, either.
My DH gets just as mad at his mom & I do, but we need to have a serious (Sunday, go to meeting talk
) before he'll say anything to her. For example, we'll be married 24 years in a couple month & he's had a grand total of one serious talk with her. Let just say, he didn't have a choice.