We can't prove either side of this, I'm not sure why you're so focused on a meaningless prove a negative thing. No one is going for proof. This is a matter of interpretation unless they release transcripts (and even then, its still not concrete). Each team will make a judgment about intent when he becomes a free agent. Each fan will make a judgment when he comes back to boo, cheer, or yawn.
That said, there's no evidence for a procedural error. None. The system has been extensively retooled and the only other reduction was based on intent, not procedure. No one involved has invoked procedural problems, which they regularly did when those were present. By contrast, Pineda and the Twins have indicated that it was due to a lack of intent defense. You can doubt the veracity of that defense (readily) but there's no indication that it was a procedural matter. At this time.
People have provided reasonable ways to prove a lack of intent. They have his urine, they can test for trace amounts of illicit substances. Diuretics and masking agents don't erase the presence of those chemicals, they just take them below the limits of testing (since PED use is about the amount of a substance in you, not the presence of naturally occurring levels). There may be a scarcity of the substances it would mask that indicate that the supplement wasn't being used to hide anything. They may also have provided a bottle from Pineda's locker and a history of openly using the supplement that indicates he wasn't hiding anything. Taking it openly would influence me as an arbitrator. They also may have interviewed the buddy and gotten his story about where the mistake was made (that would be significantly less influential to me but taken with other factors, maybe its useful?)
Well plenty of posters have suggested that his reduction implies that there must have been some evidence presented that he didn't take PED's, so yes some are posting that.
A test that doesn't show PED's just means he didn't have any in his system at that time. It doesn't prove that he hasn't taken them. He could have kept taking the masking agent long after he stopped using PED's, "just to be sure" he flushed it all out.