You have it pretty much spot on.
"To say John wasn't amused must be the understatement of the year."
John was VERY bored/unhappy.
"That is an understatement if I ever heard one"
You got the meaning correct - for example if someone had scored 100% on a test and said, 'I did OK', then 'that would be an understatement if I ever heard one'.
"He informed us with massive understatement that he was feeling disappointed."
Again you have this correct. In addition to your example, if someone had just failed an important test and then said 'I'm disappointed' when in actual fact they were heartbroken.
Hope this helps, let me know if you need anything clearing up.
Student or Learner