For all intents and purposes, you’ve piqued my interest, so nip it in the bud because it’s a dog-eat-dog world and the statute of limitations is up. You need not whet my appetite further; I’m already champing at the bit, and I wait with bated breath for your decision. If the character isn’t meant to come off as pedantic, I’d go with the incorrect but commonly-used “thing”, lest the reader be jerked from the page and onto the internet to research acyrologia. Though, really, I could care less. 😉

It depends on whether or not the character is highly educated, and whether or not you want him or her to sound pretentious to others. (Personally, I got sick of trying to teach people proper grammar.)