I'd never noticed the 'a when I read Shakespeare, and now I'm going to have to root around my shelves to look for it. "One" always sounds so formal to my ear, like it should be read aloud on Masterpiece Theatre in a cultured British accent.

"They" as a singular has been a tough one to get used to. I have no problem assigning it when gender isn't known, but the imprecision of it can get confusing. But if that's what's happening, I'd best go along with it so I don't get behind the language.