I tried sending a message to the aforementioned email address, and I got the same auto-reply of Callum John.
I've played the game of the walk in the park with the (supposed) dog, and got to the final video, that displays the date "03/20/1975". My best guess is that's the date in which the dev's wife died: according to the auto-reply email, he's started his out of office period on 03-31-1975, eleven days after the date of the video. Maybe among the convoluted collage of scenes in the video, there's something about an accident?
interestingly, there's some blood on the tree. There's blood somewhere else, on some bushes, etc. In the video there are parts of a footage shot in a forest, maybe these drops of blood in this game are linked to the forest parts, hinting to an accident happened in/near a wood?
What the hell is that thing on the right? (see also the date in the bottom left)