The MC can both read and write in Latin, although they can struggle a bit with writing. It takes them a while, and the phrases can often be incomplete or missing some verbs. It’s like a native english-speaker writing in a Romance language, basically. The entire structure goes against the “natural” way that they think.
Speaking is even harder, and if they ever do it - not counting as they’re reading, of course - they do it very slowly.
As for the second question, it is a spoiler, yes. But I can say that you’ll find its answer in this first book.