I absolutely could not cope with it either....
... Next time you go and visit them, do the separate cars thing. Don't argue, justify or explain, just take your car too. Dh takes the lead, you support. Practice the scenario - cheery smile, "no, we'll take ours too." Broken record technique if needed, smile, repeat, bulldoze.
And when you hit breaking point, have a code phrase (dh and I often do this, something you can slip into any sentence that means 'argh, I need to get out of here.') then, get in your car, and find a nice tea shop somewhere. Announce you'll see them back at theirs later, anything they need from the shops? Again, practice the scenario until the fear attached to it diminishes.
You can stay in the B&B. Just book it. You had a voucher that needed using. It's booked now, can't change it (warn the owners beforehand that your batshit crazy mil might try to cancel it.)
On one day of your visit, you announce that you've always wanted to see X. Research what's in the area beforehand, so you know what there is. Preferably a big bloody hill or something you can climb. Make sure it's something they won't want to do, but if they do tag along, no biggie because you are leading and you will take separate cars. Again, no negotiation, smile, repeat, bulldoze. Arrange to meet later. Meet up for dinner.
You are a cheery, smiley, unconcerned bulldozer with a thick Teflon hide.