I understand that you do not want a litterbox upstairs. I get it. I would prefer that as well. But... I really think FireKitty still needs one up here. I don't think her little tiny body can yet hold it in long enough to make it all the way downstairs once she realizes that she needs to go. In my opinion this has been evidenced by the fact that she has pooped in the bathroom four times now. Never pee. Only poop. I don't think she can hold her poop in.
So... while I totally understand why you don't want a litterbox upstairs, I'm pretty sure we can both agree that a litterbox upstairs is better than finding unexpected poop on the floor & in the tub. So please listen to your wife, and get over it. Dealing with a litterbox upstairs for the short term, is a lot better than dealing with a cat that hasn't been properly litterbox trained later. Trust me.
Thank you for your reconsideration of the litterbox issue.