I agree that it's difficult to know if a dog will work out based on a short meet-and-greet, especially
with a second dog!
If you keep Stella, things will start to settle back into place and you will create a new "normal." The first week is by far the most overwhelming and things should start to get better. Aside from the allergic reaction, Bubba has done all of the things you listed and he has gotten better with every single one. I have had him for almost 2 months and that first week seems so far away now.
If you bring her back to the shelter, I'm sure someone will take her. She seems like a great dog, and someone else will see what you saw in her! The guilt of bringing her back will subside, especially when she is placed into a new home.
One thing that helped me was to think in terms of time frame. How will you feel a month from now, or 3 months from now? Either you will adjust to having a new dog, or the guilt from rehoming her will begin to subside. Either way, there is a light at the end of the tunnel!
Obviously the allergic reaction is a more serious issue, and if it's not something your partner can address or live with, then that should be your deciding factor.