Then what language and alphabet exactly were used before the Romanian Latin alphabet was invented in the 19th century?
The Romanian Language was used of course.
Until about 1840, it was mostly an oral language; we had almost no literature and most of the population was illiterate.
The Romanian Cyrillic Alphabet had been in use in Wallachia and Moldavia. By in use, I mean by those very few who could read and write, most of them being priests.
In Transylvania, everybody used the Latin alphabet only.
After the introduction of the Romanian Latin-based alphabet, literacy increased rapidly; a Romanian literary language emerged and literature began to be written and published.
In a nutshell: Probably only a few thousands people ever used the Cyrillic Alphabet to write the Romanian Language in the 5 centuries it has been supposedly in use!
After Alexandru Ioan Cuza (1859-1866), the first Domnitor of Romania, more people became literate in the Latin Alphabet in any village in less than a year than were ever literate in the whole history of the Cyrrilic alphabet in all of the Romanian Lands! (school became free and compulsory in the same period)
Edit: The phonology of a Romance language, like Romanian, makes it very awkward to write it with a Cyrrilic alphabet which is designed for Slavic languages.