@Poli: The systematic study and compilation of a language by its speakers only confirms the language's long existence.
The legend (probably apocryphal) tells that queen Isabel found Nebrija's work completely useless, since everyone already spoke the language without any "help". Then she's said to have adopted it when they told her it could be used for imposing the language in the territories to be conquered.
The first texts considered the to be written in Spanish are the "Glosas Emilianenses", from the monastery of San Millán de la Cogolla. This is the first written proof that people no longer spoke local languages or Latin, but a whole new language. They're dated between 9th and 11th centuries.
The first piece of literature in Spanish is "El Cantar del Mío Cid", which was written in the 1200s.
As for Nebrija, he did compile dictionaries, but his legacy is rather centered on the grammar. If you see texts after his time, even being an established reference, spelling is not standard. Even in the 19th century, you can find some official texts whose spelling has little to do with the rules we know now.
Spelling is constantly changing. Spanish and Latin are written the way they're pronounced, so much of the spelling in previous times has changed because pronunciation changed. Many "s"-related sounds got lost and their graphic representations disappeared; some "f" sounds stopped being pronounced and became mute "h".
Spelling rules are the product of the history of the language; we keep "b" and "v" even if we don't pronounce the difference, we don't have "rebel" spelling differences between countries who pronounce "z"/"c" and "s" differently, but one cannot expect that will not change. Since the Academias just reflect how people write and speak, we have seen continuous change (maybe simplifications) in spelling for the last hundred years or so, as many written accents became superfluous ("á", "fé"), mute letters disappeared from words like when "obscuro" became "oscuro", etc.
We might expect more letters will disappear if they're systematically not pronounced by speakers, or the disappearance of "h", or maybe the replacement of "qu" for "k", or even the rejection of all written accents. We don't know.