Just got back from it. I loved it. Best of the series so far.
As far as:
Not using the time trinket to save his parents. In order to use it you had to go do your thing and then return to the point you left at the time you left. Therefore, he would have had to go back 12 years, stop the murder, and then wait another 12 years to come back and finish the spell. Assuming he still ages, that would mean he would come back around 25 years old. That's why they didn't use it for that. And probably it didn't go back that far.