what do I do if a visit is coming up and the owner still hasn't confirmed?

You need to follow-up with her immediately and say that you cannot enter her home tonight without having a confirmation from Rover. No exceptions. It is possible that she did not understand that she had to accept the booking's modification, so you might want to phrase it like that. Also, you can mention that legally it could pose a problem since you have not been authorized to enter her property until the next day and you wouldn't want her pup to not receive care.