Problems with Natural Language Change

I have two questions related to language change. First, how do you create reading rules for your language? Second, how do you create additional letters of the alphabet?

As we all know, every language changes orally, and then people agree to change the written norm. But this doesn't always happen (for example, in French, the letter H is never pronounced, but it remains in writing simply out of habit). How do you solve this problem? Do you create rules first or change the existing script?

As we all know, in the Middle Ages, the Portuguese didn't use the letter ñ, but simply wrote nn instead. But over time, to save space, they began writing the second n above the first, and eventually it became a line. The same is true for German (ä=ae, ö=oe, ß=∫s, ü=ue). Meanwhile, in Czech, all letter changes were introduced as artificial modifications (and the Poles simply use letter combinations, which, frankly, I don't think is the best option). What method do you choose for changing the alphabet?

I'm unsure about the best way to implement all these transformations.