I think that you should maybe try and keep yourself calmer regarding the helpful MIL. She is not showing any disrespect towards your housekeeping style, but just this way inclined,with wanting to keep busy,busy.. It really doesn't matter what jobs she tackles, unless she goes forth,and completely re arranges your entire house.
My late Mum , when visiting,used to like being busy and helpful in my home, but she did always ask,if it was ok,to do this and that.I just used to use this Mum busy time,and go and do stuff in my garden,,my favourite thing.And in the mean time, she had done all sorts of jobs. even some ironing,which I never did,, and she had to scrabble in the depths of my shed,and search for the ironing board,which was covered in cobwebs and spiders.
My late mother in law though,would visit,and just sit drinking cups of tea, eating biscuits, and smoking,,and maybe talk to our dog, who we asked her not feed with biscuits,which she often tried to do,