Is it permissible for customs to be overturned, such as a barley grain transforming into a human, or a human coming into existence all at once without upbringing?

You mentioned that what you asked about is rationally permissible, even if it is customarily impossible. The impossible is divided into four categories: impossible by addition, impossible in existence, impossible according to our mental structure, and absolutely impossible. What is impossible in itself is that whose existence the mind cannot conceive, like combining two opposites. What is impossible due to something else is that whose existence the mind can conceive, but custom has not allowed its occurrence, such as flying and walking on water.