Complements of degree that aren't extreme?
Let's take a look of the first example:


  高兴得不得了


The first 得 is a structure of emphasis. It can be translated into English structure "So A that B": "So 高兴 that 不得了（了）". Literal meaning of “不得了” is some extraordinary thing happens. For example "不得了了，房子着火了" (House is under fire). So the literal translation of 高兴得不得了 is "So happy that the happiness is extraordinary". And "so x that x" in English is never used for mild extend, instead it is for extreme extends.

Other adjectival complements are similar. You might only want to express your happiness, when the level of 高兴 exceed some kind threshold. If it is not significant, you might not even bother saying you are happy.

I will try to give you another example. Say you are dragged to a party by your roommate, but you don't want to go at all. You expect it to be super boring, but when you actually arrived there, you feel a little happy. You are surprised by this feeling: it is not what you expected. Now you want to say you are 'a little happy'. You can say "我居然有一点高兴。" Now that "有一点" makes sense. If you want to decorate the sentence to show more ironic emotion, you can say "我居然有那么一点小高兴".