I think you are missing the most critical features, stability and support. You need whatever distro you pick to be solid. No one cares about the file system or whether it is immutable or not, your users need the computer to work when they use it, and nothing else matters.
You also need to be confident you can update and upgrade safely and easily, any risk of a broken update will make your life a misery when that happens.
Kickstart support, or some form of automated deployment will also be extremely valuable so that you can easily redeploy broken boxes with minimal effort. And some form of remote config/admin will also be extremely valuable. You dont want to have to do updates manually one at a time.
I would pick a general purpose commercially backed OS, so that if you need it later, you can pay for support if there is a problem. And you need to write some basic usage guides, because no matter which distro you pick, if its not Windows, your users will complain when they cant do X the same way it works on Windows.