>- What decision-making process led to the idea of injecting human urine into a frog in the first place?

such an approach has long history - peeing on wheat is known from the times of Ancient Egypt and it was widely used in Middle Ages too

https://history.nih.gov/illustrated-histories/thin-blue-line...

"Bastard Executioner" series set in 14th century has a scene on using several objects to test urine for pregnancy on.