Another discongruency that's always interested me is the phrase "the two of them in the euphoric bitter air." Can something be euphoric and bitter at the same time? I'm trying to imagine it.
I have two possible interpretations for the "euphoric, bitter air". The first one is more mundane, the second one more spiritual.
The mundane one:
Let me start with the second word, bitter: I always interpreted it as bitterly cold. Now I looked it up in the dictionary and indeed, it said that bitter, when used in regard to weather, wind or air, means bitterly cold.
So although it was bitterly cold on the mountain, and other people would have hated the situation and the job, they didn't mind it - in fact they loved their time up there. They loved it so much, it gave them such a sense of euphoria that they felt like they were able to fly.
The more spiritual one:
The air was euphoric for the same reason as before (liberating, clean, giving them so much sense of freedom and happyness they thought they could fly). And the "bitter" part is meant as a foreshadowing of the things to come and as a reminder that even in their Garden Eden, the world was not perfect: Aguirre watched them (and this is told to the reader in ther very next sentence!), and both knew that they could not stay on the mountain forever, that everything has to come to an end.
I can imagine things that are euphoric and bitter at the same time (bitter
not in the sense of cold, but in the sense of hard/ difficult/hurtful). BBM-cat's example is a good one. Or imagine a person who was once very important and/or beloved to you, but you parted in anger - and then meeting this person for the first time after years. Think also of the word bittersweet.