He can't do ANYTHING, there are still limits regarding expended energy.
Listening to the book makes it harder, but I am still fairly certain that you contradicted yourself. Either there are 2 languages, and in which case Eragon did his ballad in the elvish one, or there is just the one language that elves speak which is the ancient language and it can be used to create fiction.
No, it's made clear that Eragon wrote his ballad in the Ancient Language. Oromis says he should not have been able to do so, and Eragon says he was because he believes it to be true with all his heart(or some such).
The Elves didn't come up with the Ancient Language, some other race did. The Grey Ones, or something.
I still say there's the intent to deceive which it blocks you from acting on, not necessarily telling untruths.
But we are given no explanation for it.
And Oromis wouldn't have taught Brom the secrets if Brom didn't finish the training (Which Brom didn't) because Oromis says they only taught the unspoken spell thing to students who had mastered every bit of magic. Which Brom hadn't.
Why didn't Brom finish training? There's no reason for him not to have. He and Morzan trained together.
Morzan apparently did, or was he an untrained apprentice-type dragonrider who somehow managed to kill a bunch of other fully-trained Dragonriders?
Another thing that bothers me:
We are told that casting magic without the Ancient language is incredibly dangerous and that the Elves are taught not to do so unless it is absolutely necessary.
Yet the Queen non-verbally magicks up some flowers when Eragon meets her. And Vanil uses magic non-verbally in ever one of his duels with Eragon.
And another thing: Eragon's swordsmanship. In less than a year Eragon has gone from being a farmboy to being able to best a warrior with literally over a thousand years more practice than he.