They need evidence to arrest... Police can't arrest people for not answering questions
No, you seem to have difficulty understanding the basic concept (the old Davros mantra).
If you're so certain they 'wanted to arrest / charge' her, then they could have. They could have done so (while they were torturing her) in the presence of her lawyer. Because them being at least cognisant of the fact that arguido status allows silence as opposed to compliance, despite them being cast as total idiots, they would
know that there was a high probability that she would be advised of that course of action.
Once the ground rules had been established, why do you think they were going through the motions of asking 40 more questions, knowing the response they would get? At that point they're not establishing any facts, they're establishing process and is evidence in itself.
Edit: Besides, plea bargain or no,
I believe frank, informal discussions were held prior to the interview with all parties - I'm not saying 'plea bargain', but something went down that made Amaral so very, very sure of his case.