What happened to Milton?
Solution 1:
There is an answer for what happened to Milton, but this answer is not found specifically within the story of What Remains of Edith Finch. Understanding Milton's story requires familiarity with the developer's previous game The Unfinished Swan. Creative Director Ian Dallas confirmed in a reddit AMA that Milton is more-or-less canonically a character in that game, which has unusual implications for Edith Finch's world.