Two wrongs don't make a right. I think the LW was rude for retreating to her bedroom for the entire night.
That said, I suspect this has probably been an on-going problem with the MIL, and the LW might well be at her wits end. I also think that putting the responsibility on the MIL to help feed the extra guests wouldn't work. I can imagine the LW asking the MIL: "Hey, I've only catered for four people, not eight. Can you please pick up some more food and wine on the way here?" and being met with a response of "Oh, don't fret, there'll be enough to go around." or "You and I can just eat a bit less, to stretch it out!"
If I were the LW, I would have welcomed the extra guests, but would have made a comment along the lines of "It's lovely to see / meet you, but I actually didn't know MIL had invited you until 30 minutes beforehand. I apologise in advance if the food's a little light on." I think saying that in a polite, cheerful tone would be ok.