It sounds like you have discussed and thougth about a lot of things already
If you and your DH want this to be a "short term" arrangement I think you should also spell out ahaed of time what short term means (you may think 1 year, DH may think 5 and DMiL may think 10 .....). I think you know this, but jsut to be clear: I am actually
not trying to dissuade you. I have always told DH that his mother ever need to live with someone (right now she and his dad are fine but his dad does have heart conditions which frighten all of us into believing there will come a day when he is suddenly gone) I would absolutely take care of her if she will move to where we are. We both know I am much more likely to take that on than any of his siblings or their spouses. I adore my mother in law and would want to help her much like you want to help yours. I am just trying to help you think of everything ahead of time ad get everything handled as well as possible so that at in the end you are still as close to your MiL as you are now.
Smoking: as an adult you are totally correct she has the rigth to chose to smoke. As a Mom, one I would be worried not only about the effects of second hand smoke but about the example it sets for my chidlren (especially young ones like your 2 year old) if they see her outside smoking day after day.
TV: If she does not end up with her own apartment I would worry about the kids being exposed to news all day long (lots of violence there). Maybe if you present it to her like that she will be more likely to limit viewing.
Furnsihings: Will she want to bring her own stuff with her (to feel at home)? how will that fit in with what you have if you cannot swing a seperate apartment? Will the smoke smell in it drive you batty?
Discipline: If she is babysitting a lot and gets used to dealing with discipline, do you want her doing so also when you are there or stepping back unless hse is the only one? When does she get to be hte doting grandma if she there all the time?
Moving: what if your husband gets offered a promotion but you must move to take it? Will you feel free to do so or will you feel tied to the area because of teh shared living arrangements?
Houseguests: How will it impact your MiL if you have house guests? Will your family still be able to visit? How will YOUR mother react to you living with your mother in law?