I agree with everything you said in the paragraph I bolded. I think Brooks was given a solid opportunity in his rookie year to show what he had at that point in his career and he had his ups and downs. In his decent amount of playing time that year, I believe he made around 20 tackles, a couple pass deflections, a fumble recovery, and obviously that big hit on Delanie Walker. He also made around 2 or 3 "bad" plays that year. The two I can remember were similar and were the plays against the Bengals and the Saints. He was in position on both of them, but once the ball got in the air he didn't make the right decision on the ball. This is why I'm calling them "bad" because he didn't completely blow those plays, but he just couldn't finish them. I think those mistakes he made are very fixable and are part of the mental aspect that you mentioned. Plus I don't see how only making 2 or 3 mistakes as a rookie safety is a that bad of a thing, they are more like learning opportunities. I'm sure he learned from those mistakes he made and hopefully he'll make the play next time in those situations when he gets the chance.
Injuries have probably been the biggest factor of his issues. The injury that happened during his rookie year was a very serious injury. It happened near the end of the season which is good and bad, but he only missed a couple of games because of it that season. I honestly think that injury had more of a negative impact on his second season than his first. He came back remarkably quick from this injury which was awesome, but I don't think he was always 100%. I also think that the injury affected his chances to start last season because he wasn't able to participate in the off-season program and I believe his uncertainty was part of the reason we went out and signed Kendrick Lewis. His thumb injury last season wasn't as bad as the other one, but it also came at the wrong time. Before the injury he was starting to play well and get more playing time game by game. Then what's really unfortunate is that he missed the opportunity to take over the starting role for at least a game and a half where Kendrick Lewis had his minor injury. Brooks has definitely had some bad luck in the injury department. Lets hope that doesn't continue.
Also, I think Eric Weddle could be a great help to Brooks. He's a proven safety in the league and has a similar skill set to Brooks.