There have been some of our members who have returned to their homes alone after surgery---I will tag
@Josephine, as she can tell you about how she handled it.
If you are well prepared, you should have very few issues.
Am I unrealistic to think that after a few days I will be able to manage alone?
Not at all. I would prepare meals ahead of time and freeze them, and you can also have some of your friends, neighbors, or church-goers help you, too.
I do have friends that can take me shopping, etc.
I would not plan on going shopping much at all in the early days after your TKR---you will be mostly sleeping, icing, resting, and elevating, and going out too soon---doing too much, too soon, while a quick shopping trip may seem harmless, will cause some serious inflammation. I would get all of the shopping done ahead of time and then just have the neighbors help out and go for you if you need some "fill-in items."
If your brother can, I would have him there for the first several days after you return, just to help you get things settled and to check up on you. I would also have the neighbors, if the can, check in on you later on.
It sounds like, logistically, you are in great shape with the kitchen and the bathroom already on the first floor as well as fitted with what you will need.
If you prepare ahead of time, and with some neighborly help here and there, you will do just fine.