I'd say the first one. Miguel seems to always focus on what has happened in the past, and not just the justifications created by the family history (which he narrates at the beginning). He looks up for an historical figure - Ernesto De La Cruz - to fuel his passion / drive to play music, which is at odds with the traditions in his family and sets him apart from the rest : he aims to follow in his footsteps by following his dream, just like De La Cruz (and Hector) did decades ago. A mean to repeat the past that puts him in constant struggles with his surroundings, making him eventually push his family away ("I hate being part of this family", "I don't want to be remembered", etc.) That, and his whole quest is built around the false premise that De La Cruz really is related to him because his true great-great-grandfather has been erased from the family's history. And later in the movie, him realizing that Hector really is his ancestor and that De La Cruz poisoned him for glory (we and the characters understand what has truly happened) leads Miguel into even more troubles as he is thrown into a cavity. But in the end, he's the only one who is able to shed the lights on De La Cruz and set history right.
Again, I believe this ending to be more a matter of a Changed MC (Memory) than a Steadfast MC (Past), considering that the ending isn't just re-evaluating what has happened, but remembering someone for who he truly is (having Coco remember her dad, Hector being remembered as the true composer of the songs, etc.) In any case, Memory plays an important part in the movie so I believe it to fit somewhere in the Concerns.