So many people do not understand the fundamental basics of threat in this game that it is a wonder tanks are given a hard time over holding aggro.
As for guarding the healer, if they are too far from the tank, it is useless to do so, and you are better off guarding the higher dps.
In short, learn the mechanics of the game, all classes and roles, to better handle the situations. If you don't, then it's a good deal that you are in error.
Take some of your own advice here as you are incorrect about how the Guard function works.
So far this thread has been full of some great info side by side with people that are just flat out wrong.
Unfort. for the new or casual player it will be difficult to tell which is which.