Honestly, sticking to your guns, even if you fall out, is the quickest way to nip all this nonsense in the bud.
They're testing you - mainly you, but also their son. Where do his loyalties lie? Are they still in the 'boss' position now he's married and an adult? (this mainly FIL, by the sound of it). They want to flex their muscles and show both of you that they, the real grownups and the real 'parents' are still to be deferred to.
Nope!
I would agree with what everyone above says, and make one extra point. In the long run, it's easier to get on than not. What's the easiest way to get to that point? - by, if you need to, falling out now. If you set a boundary and cheerfully stick to it, their choices are to suck it up or lose out. If they see you mean business and are really quite happy not to see them if they strop off, the chances are they'll get to grips with that and you'll be able to have an ok relationship. Cave, or pander, and this kind of scenario will just happen again and again and again until you really really do hate them so much that there will be no way back. This seems to be what's happened so far - you sucked up their rudeness, the house rearranging... and so here they are, trying to bully you again because they think they can.
It's great that your DP is on side by the way. Lots of men aren't - little do they seem to realise that by not setting boundaries with their own parents, all they're doing is facilitating year after year of tantrums, bullying, and push push push.
So. Cheerful reply 'Oh well that's very sad but we can arrange to do something another time.' Snarky comments, get DP to reply 'No, we aren't being unreasonable - how would you feel if we tried to tell you what we'd be doing in your home after you'd said no? Have some manners, please.' If they come, and ignore you - ignore them, and make sure their ignoring means that they get to feel isolated and excluded - don't pander. And yes, when they leave, your DP says -'Well mum and dad, that was embarrassing for me! Really didn't think you'd be so rude to my friends and family. Probably better we just don't invite you to future celebrations if you're gonig to sit there like sulky toddlers.'