I suppose that he's getting such a long prison term more because of his insanity than because of the child diddling.
Saul Goodman attended AO's deposition. Though he (thank god) declined to give us the specifics, he said that Nick had apparently committed certain types of acts on her that went way beyond what was in the AIM confession. It's unfortunately easy to guess the sort of things Nick probably unleashed on AO.
For example, Nick was desperate to make people connect the idea of him not being able to get a boner from feces with the idea of him not being a pedophile. This is completely illogical if we're assuming only the chat logs were used as evidence, but they were not. Nick had been reacting to his sister "telling" on him. So, essentially, he was more likely trying to prove that AO's abuse story was false because he wasn't into feces.
The conclusions we can draw from this theory, combined with the knowledge that he did a lot more to AO than he told Anna about on AIM, are the stuff of nightmares. I feel genuine hatred for Nick at this moment, where before I was trying to be emotionally uninvested. I'm sure it's only a small measure of what the whole court felt for him when they had to listen to AO's story.
His prison term is not because of his insanity, but because of just how horrendous the things he did were. I have almost never heard of a first-time offender getting a sentence like Nick received, and it's quite clear that he got it because he committed truly evil acts against his sister. I'm glad Saul didn't actually repeat what specifically happened to AO, but it's not that hard to figure it out if you know the kind of person Nick Bate is.
I'd better stop before this turns into a-logging.