A key point is that the murder is done by a longbow, but the arrow passes completely through the victim, lodges in a tree, and is not detected until much later.
This strikes me as implausible. Can a longbow arrow really do that? A human body contains a lot of tough materials, up to and including bones. My physics mistake detector is flashing here.
But I am not an archer. Anyone have real-world experience?
It’s called a pass through shot, and it does happen with deer.
May I presume that it would be a pretty lucky shot, not something that the archer/murderer could have planned on?
And then I wonder whether the likelihood of getting that lucky was part of the plot. I think I read that book but it was a long time ago and I don’t remember much about it.
Did the shot go through the the abdomen? As long as it misses the spine, that would seem possible. I know nothing about the velocity or penetrating power of an arrow shot with a longbow, though.
[Googling] Multiple sources say an arrow can indeed go completely through a body, with the right kind of arrowhead shot from a powerful enough bow. Medieval longbows could shoot an arrow through chain mail, though not good-quality plate armor.
ETA: Here’s a Reddit discussion of just how lethal bows and arrows can be, complete with links to different types of arrowheads.