- First, I would like to point to an inconsistency between Chapter 71 and previous chapters: When Keith Shadis finds Eren Yeager after Eren become a Titan for the first time, he finds only Eren, with no sign of Grisha, whereas in previous depictions of Eren's first Titan transformation, he eats only the upper half of Grisha's body, leaving Grisha's lower body (and right hand) untouched, in plain view for all to see. Now, admittedly, comic books, manga and the like are notorious for minor errors such as these, but I belive it was a deliberate choice on Isayama's part to hint to fans that Grisha's story has not yet ended.
- Second, Grisha is Titan Shifter, who we have seen are notoriously hard to kill. In fact, the only Titan Shifters we've seen to be successfully killed are members of the Reiss family (each of whom were COMPLETELY devoured, with no remains left over to regenerate).
- Third, in Eren's memories, Grisha is always seen injecting Eren with a vial of mysterious serum. after everything we have learned from the Reiss family, it has become obvious that what Eren was given was the same serum all of the Reiss family members are seen injecting into themselves so that they can become Titans and consume their predecessors.
- Fourth, when Shadis finds Eren in the woods, Eren is unconscious, meaning it is very likely that Grisha was able to regenerate with his Titan abilities and leave the scene without being seen, explaining why Eren has no memory of it.
- Fifth, (most importantly) it has been stated that the Coordinate and Titan Shifting ability come from the fluid in a human's spinal column, so theoretically Eren would not have to completely devour Grisha to take the Coordinate from him, he would just have to devour Grisha enough to get the Coordinate out of his spinal column.
With all of this, I would like to give a brief summary of what happened (in the order that it happened):
- Grisha learns of the Reiss family, and the mysterious power "The Coordinate" that they possess.
- On the day of the Colossal and Armored Titan's attack, Grisha left his family, telling them he was going on a trip, but not telling them that where he was going.
- Grisha arrives at the Reiss family's chapel, and uses his Titan Shifting ability to kill them, and take the Coordinate from Frieda.
- Grisha returns to Wall Maria, where he meets up with Shadis and Eren, and upon hearing that Carla has been devoured, takes Eren into the woods, instructing him to take revenge upon the Titans.
- Upon arriving in a clearing in the forest, Grisha injects Eren with the same serum used by the Reiss family to gain their Titan Shifting abilities, causing Eren to transform and partially devour Grisha.
- After regenerating, Grisha finds Eren unconscious, and, satisfied with the results, leaves Eren, who is subsequently found by Shadis.