The XML editor in Eclipse has, unfortunately, a long history of flakiness. It's better now than it used to be, but I still have occasional problems with it - most recently while editing JSF faces-config files. As for Intellisense getting into the act, I think they need to resolve the deeper issues, first.
Education won't help those who are proudly and willfully ignorant. They'll literally rather die before changing.
I occasionally run into the intellisense not working issue. I have used two mechanisms to "fix" this.
First, create a new workspace and move your project to that workspace. This will fix problems where the workspace config settings get messed up.
If that does not work, reinstall Eclipse.
By the way, I suspect that letting Eclipse do an update of its plugins often causes this issue. I never do updates any more and thus have not had this issue for a while.