re: so the **** hit the fan, …
so let me get this right…this is not the first time that it has come to your attention that the “kaamwali” prefers not to wash undergarments and yet you felt it was okay to give her your own?
men rarely wash any article of their own clothing while women often wash their own. my mother used to wash one outfit…right there in the bathroom/bathtub while she bathed. I don’t recall her ever having enough laundry to fill a machine load.
MIL is not being a drama queen then… you are. She has never spoken to you directly about anything so why would she start now? Furthermore, she might have felt that speaking behind your back to your hubby does not achieve the desired results so she thought to have a ‘group’ discussion. 
I have a 9 year old and even she knows by now not to put her unmentionables in the hamper in the family bathroom. She puts them in her own private hamper, as does mama…and that too covered with some other article of clothing rather than just lying right on top visible if the hamper is opened.