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?
Also:
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?
And also:
What the hell is that thing on the right? (see also the date in the bottom left)