It's just one day. Next time ur chatting with MIL just mention that you will have spare knickers next time in case there are any accidents as u would prefer nappies weren't used.
Unless you had previously said to her that u didn't want nappies used, how was she to know. And you DH..if he hasn't been greatly involved in the TT process, it's understandable he would just agree to what ur MIL suggested.
They gave you a break for a day....I think you are overreacting big time - sorry!