How can one reconcile the fact that knowledge of the time of rainfall is one of the five keys to the unseen, with weather forecasts that specify the time of rain?

What is mentioned in the fatwa is correct. Forecasting weather conditions is not part of the knowledge of the unseen (Ghaib), as it is based on understanding God's cosmic laws, scientific experiments, and conjecture, not on certain knowledge and conviction. What is determined by calculation is not divination, such as predicting a solar eclipse or a lunar eclipse. Similarly, forecasting weather conditions relies on observable phenomena.