I'm not a psychologist, nor do I play one on TV, but I got the impression the LW might have some social anxiety issues. She mentioned that she had to have time to deal with the fact that she was having guests.
While I don't think retreating to her bedroom was appropriate under normal circumstances, this might have been her attempt to tamp down the anxiety. MIL showing up with uninvited people may have thrown her off balance.
In any circumstance, her husband should have backed her up and told his mother that they couldn't accommodate the other people.
I had a great aunt who was infamous for this. When my mom's youngest sister got married GA and her husband were invited to the wedding. She showed up with two of her adult children as well because "I knew Donna would want Paul and Carol to be here". No, if Donna wanted Paul and Carol to be at her wedding they would have gotten an invitation. The newlyweds were charged for two additional dinners. When I got married, Mom headed GA off at the pass. She was firmly told "BeagleMommy can not pay for extra guests. You, and only you, are invited (husband was deceased by this time). Do not bring anyone else."