I'm guessing the victim was abusive to his wife, Houka, given the sunglasses. Someone last time suggested that the woman in the wheelchair, Himi, is likely the one referenced by the code, and she was said to be best friends with the wife. So it's likely she killed him as revenge.
As for why he used the cipher instead of just text, maybe he was worried Himi would see it? They were searching together after all. Alternatively, he may have been concerned about their ability to fight back if Himi attacked since she's likely faking her injury.
No idea what's up with the missing pine tree, except it may have been involved in Houka's death/disappearance. The Ace nickname is also weird. I'm guessing it's a reference to when they'd play cards as kids, but no idea what's up with the "you're the real Ace" comment to Hejima.