They tried getting her to write but she couldn't for some unexplained reason.
I don't think it's that she couldn't write. It's that it wouldn't have been a very smart move to write anything.
We don't really have a timeline for when she started to change her mind about that. Perhaps sometime before the reveal that she could speak. By then another outbreak had occurred anyway
And yeah, I think the whole vocal cord parasite thing was stupid. Although, parasitic vocal cord infections are real. But have nothing at all to do with languages.
As for mute characters. This reminds me of that episode of Buffy The vampire slayer when all the character's voices were removed and the entire episode was done in silence. It was actually really interesting.