If you put your Chromebook in Developer Mode, you should be able to access a root Linux shell and remap buttons there. However, I don’t think Chrome OS uses Xorg at all so your xinput solution won’t work to remap mouse buttons. The only solution that’s effectively guaranteed to work is to install Crouton and run a Linux distro of your choice on it. You would still have to deal with the annoying boot sequence and slightly less security.