I did a test post with double and single quotes. They were still curly or smart.
WordPress makes the singles go backwards too when they should be the other way.
WordPress has been stupid about this, arbitrary, even bossy about it by not having had an option built in all along, at least for years now. They've known about it and just ignored the hundreds and even thousands of people who've asked for the option in one way or another (plugin downloads, etc.).
It's like WordPress is refusing just to spite people. It's been way to noticeable not to be part of the reason.
I know. I'm venting. Oh well.
I hope you get your plugin working and can keep it working. Other plugins have work for a time, but WordPress always manages to break them and still refuse to offer the option.