How are Park Sang-Gun's letters able to work? One of the strict rules of the letter-altering power is that the letters must describe objective reality or it won't work; however, it's later revealed that Park Sang-Gun is an Unreliable Narrator and that in particular his illness made him wrong about key details of basically all his first few letters. Shouldn't any effort to rewrite fate using them have failed in the same way Myth's letter failed near the end? His father never beat him, so both letters that describe it happening should be invalid for rewriting history.
The simple answer is that the letters can contain subjective world view but the events themselves objectively happened. So at most the events happened but the "father" and "mother" got flipped.
