Maybe it's only my utterly spoiled children, but they never did discomfort well.
They would last ONE night in this scenario and be totally allergic.
They are polite children, but they would hate it.
They would not like a room with so many other children sleeping on the floor.
They wouldn't sleep well.
They would be like a bag of cats after 2 nights.
We live a distance from some family and have only EVER done one night with them.
We booked hotels nearby and everyone had their bed, sleep, and were happy.
You are spending huge money to fly there and it is indeed likely the holiday will be ruined.
Being able to leave at the end of the day and have some sleep and rest could be the thing that makes this time bearable.
Your MIL crying if she is challenged is such a recipe for stress and upset.
I really think, in your place, I would book the Premier Inn NOW.
Say nothing and just tell her a month beforehand that the children haven't been sleeping well and this is for the best.
No discussion, a fait accompli.
If she is hysterical, let her work it out of her system.
She sounds like such a manipulative bully.
In this situation, your children getting decent rest comes ahead of her tantrums.
I predict a disaster holiday that your children will never want to revisit if you stay with them for the week.
Her tantrums don't trump your childrens welfare.