What is the ruling on my fasting, and do I owe expiation if I drank water and used a nasal spray a few minutes after the Fajr اذان (call to prayer), thinking that it was still permissible?

You must make up for the day you drank after the dawn, thinking that the night was still remaining. You are not required to offer expiation or feed the poor, nor is there any sin upon you, as the act was not intentional.